Footage posted by US Central Command showing weapons system video of a US airstrike against an ISIL compound northwest of Ar Raqqah, Syria.
The US and Arab strikes on militant targets in Syria overnight were "only the beginning" of a "credible and sustainable, persistent" coalition effort to degrade and ultimately destroy ISIS militants and other extremist groups, the American military has said.
The airstrikes - which employed US Tomahawk missiles, B1 bombers, F16, F18 and F22 strike fighters and drones - was backed by support from Saudi Arabia, Bahrain, Qatar, Jordan and the UAE - a coalition of nations that has agreed to assist with the destruction of ISIS.
There was also a separate US attack on a different band of Islamist militants in Syria - the mysterious Al Qaeda-affiliated Khorasan Group, who are said to have been planning an "imminent attack" on a Western target.
"I can tell you that last night's strikes were only the beginning," Rear Admiral John Kirby, a Pentagon spokesman, told reporters. He said the strikes had been "very successful" and would continue, without going into further detail on future operational plans.
Another military spokesman, Lieutenant General William Mayville Jr, said that Arab nations - including Saudi Arabia, Jordan, Bahrain and the United Arab Emirates - took part in the second and third waves of attacks. He said the Arab countries' actions ranged from combat air patrols to strikes on targets.
Earlier US President Barack Obama said the participation of the five Arab nations "makes it clear to the world this is not America's fight alone."
Before an after photographs of an ISIL storage facility near Abu Kamal, Syria, struck by US strike aircraft. Photo / US Central Command
Speaking a press conference just hours after the overnight raids, Obama said the joint fight against ISIS will take time - but remains vital to the security of the United States, the Middle East and the rest of the world.
He said it was not possible to know how long US-led operations against Islamic State militants in Syria and Iraq will last. In a letter to Congress following the air strikes, Obama said: "It is not possible to know the duration of these deployments and operations.
"I will continue to direct such additional measures as necessary to protect and secure U.S. citizens and our interests against the threat posed by ISIL," using another name for Islamic State.
He added that the US is "proud to stand shoulder-to-shoulder" with the Arab nations in conducting the strikes.
Obama's comments came after America's top military officer Army General Martin Dempsey said the overnight airstrikes will have successfully shown the terror group that its attacks will not go unanswered, and that even strongholds such as the city of Raqqa do not represent a "safe haven".
Air strikes were launched from land bases in the Middle East, while the F18s were launched from the aircraft carrier USS George HW Bush, which is in the Persian Gulf.
Tomahawks were fired from the USS Arleigh Burke, a guided missile destroyer in the Red Sea.
The first wave of strikes lasted for 90 minutes and one video on social media showed blasts across Raqqa - the de facto capital of the Islamic state.
Among the targets which reports from social media claimed had been hit were the house of the governor of Raqqa, the national hospital and the Equestrian Club.
One account said at least 20 of the militant group's fighters had been killed in the strikes, with other sources saying "dozens" were dead.
Separate ISIS targets were hit in Deir al-Zor province in the north of Syria, where the terrorist group has also gained vast swaths of territory.
"We wanted to make sure that ISIL knew they have no safe haven, and we certainly achieved that," Army General Martin Dempsey, chairman of the Joint Chiefs of Staff, said in an interview with American reporters. ISIL is an alternate acronym for ISIS commonly used by US officials.
America said five Arab nations either participated in the airstrikes or provided unspecified support. They were Saudi Arabia, Qatar, Bahrain, Jordan and the United Arab Emirates.
Dempsey said the role of the five Arab nations in the airstrikes was indispensable to the US goal of showing that the battle to degrade and defeat the Islamic State group is not just a US fight.
"I can't overstate" the importance of the Arab role, he said, calling it an unprecedented coalition with Arab states and said the partnering has set the stage for a broader international campaign against the extremists.
Dempsey said the five Arab nations' agreement to join in the airstrikes came together quickly; as recently as Sunday but did not specify exactly what role each nation played overnight.
He told reporters that more Arab participation was needed before President Barack Obama would sign off on the strategic air campaign.
The US also carried out separate airstrikes against Al Qaeda-affiliated extremists Khorasan in Aleppo and Idlib, following intelligence that the group were planning an "imminent attack" against Western interests.
The US said at least 30 fighters died, along with eight civilians, including children, in Aleppo and Idlib.
Khorasan are a totally separate group to ISIS and, although little is known of their origins, they are understood to be made up of "seasoned al Qaeda veterans".
Lieutenant General William Mayville, the Pentagon's director of operations, said Tomahawk cruise missiles fired from US warships in the region were used to hit the group's compounds, workshops and training grounds around the city of Aleppo.
"Intelligence reports indicated that the Khorasan Group was in the final stages of plans to execute attacks against Western targets and potentially the US homeland," he said.
It is understood that the group's aim is not to fight against Syrian president Bashar al-Assad, nor does it hope to acquire vast swaths of land like ISIS.
Instead Khorasan targets the thousands of young Western Muslims who have travelled to fight in Syria in recent months - many of whom have retained their passports and have made the journey without the knowledge of their home governments.
Khorasan's plan is to recruit and train this fighters not to carry out attacks in the Middle East, but to return to their home countries and commit catastrophic acts of terror there. It is understood that once such attack was "imminent" which led to the US targeting the group in air strikes overnight.
James Clapper, the US Director of National Intelligence, said: "In terms of threat to the homeland, Khorasan may pose as much of a danger as the Islamic State."
Pentagon press secretary Rear Admiral John Kirby said the decision to launch the strikes was made on Monday, only hours before US Air Force and Navy pilots were given their missions and targets.
Kirby said the strikes were ordered by Army General Lloyd Austin, the commander of US forces in the Middle East and South Asia "under authorisation granted to him by the commander in chief".
This morning British Prime Minister David Cameron released a statement saying he supported the strikes against ISIS by the US and allies, and will now discuss what contribution the UK can make.
Kirby did not name the partner nations participating in the operation but they are now known to be the five Gulf nations and Jordan.
All of them are majority Sunni, the same branch of Islam as ISIS. Britain and France were not involved in the strikes.
News of the airstrikes comes as ISIS' highly-organized press office released a second propaganda video appearing to feature captured British hostage John Cantlie.
The journalist - wearing a Guantanamo Bay-style orange jumpsuit - is filmed questioning America's preparation for attacks on ISIS, and compares the 'unwinnable' conflict to the Vietnam War.
In the five-minute scripted video, Cantlie suggests Barack Obama, long careful to avoid the sort of conflicts his predecessor George Bush pursued, is being sucked into an "unwinnable war".
"The president once called George Bush's Iraq conflict a "dumb war", and couldn't wait to distance America from it when he came into power. Now he's being inextricably drawn back in," Cantlie says.
Military officials have said the US would target militants' command and control centers, re-supply facilities, training camps and other key logistical sites.
An anti-militant media collective called 'Raqqa is being silently slaughtered' said among the targets were Islamic State buildings used as the group's headquarters, and the Brigade 93, a Syrian army base that the militants recently seized.
Other airstrikes targeted the town of Tabqa and Tel Abyad in Raqqa province, it said. Their claims could not be independently verified.
According to ABC News' Luis Martinez, the stealth F-22 Raptor fighter jet saw its first combat ever with the Raqqa strikes.
Weapons system video of US airstrikes against an ISIL compound northwest of Ar Raqqah, Syria. Photo / US Central Command
The Syrian government of Bashar al-Assad was said to have been 'informed' of the strikes, but not asked for authorisation.
Syria's Foreign Ministry told the Associated Press that the U.S. informed Syria's envoy to the U.N. that 'strikes will be launched against the terrorist Daesh group in Raqqa'.
Daesh is a name for ISIS used by many Arabic-speaking media organisations.
Former Delta Force officer James Reese told CNN: "This is the punch in the nose to the bully that we talked about on the playground. ISIS is the bully, and we just punched him in the nose."
Defense Secretary Chuck Hagel said that the plan "includes targeted actions against ISIL safe havens in Syria, including its command and control logistics capabilities and infrastructure." He said he and Dempsey approved the plan.
The US has also been increasing its surveillance flights over Syria, getting better intelligent on potential targets and militant movements.
To date US fighter aircraft, bombers and drones have launched about 190 airstrikes within Iraq.
Urged on by the White House and US defense and military officials, Congress passed legislation late last week authorizing the military to arm and train moderate Syrian rebels. Obama signed the bill into law Friday, providing $500 million for the US to train about 5,000 rebels over the next year.
US leaders have also been crisscrossing the globe trying to build a broad international coalition of nations, including Arab countries, to go after the Islamic State group and help train and equip the Iraqi security forces and the Syrian rebels.
The militant group, meanwhile, has threatened retribution. Its spokesman, Abu Mohammed al-Adnani, said in a 42-minute audio statement released Sunday that the fighters were ready to battle the US-led military coalition and called for attacks at home and abroad.
Military leaders have said about two-thirds of the estimated 31,000 Islamic State militants were in Syria.
In a speech September 10, Obama vowed to go after the Islamic State militants wherever they may be. And his military and defense leaders told Congress last week that airstrikes within Syria are meant to disrupt the group's momentum and provide time for the US and allies to train and equip moderate Syrian rebels.
"I have made it clear that we will hunt down terrorists who threaten our country, wherever they are," Obama said. "That means I will not hesitate to take action against ISIL in Syria, as well as Iraq. This is a core principle of my presidency: if you threaten America, you will find no safe haven."
The US military has been launching targeted airstrikes in Iraq since August, focusing specifically on attacks to protect US interests and personnel, assist Iraqi refugees and secure critical infrastructure. Last week, as part of the newly expanded campaign, the US began going after militant targets across Iraq, including enemy fighters, outposts, equipment and weapons.
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