Iran has threatened to strike inside the United States after launching a series of missile attacks on Iraqi bases housing American soldiers.
Tehran officials said the attacks marked the beginning of a promised retaliation against the United States for the killing of top Iranian military commander Qassem Soleimani.
In a statement, Iran warned it would attack inside the US if the Trump administration responds to the missile attacks.
"The fierce revenge by the Revolutionary Guards has begun," Iran's Islamic Revolutionary Guard Corps said on a Telegram channel.
"We are warning all American allies, who gave their bases to its terrorist army, that any territory that is the starting point of aggressive acts against Iran will be targeted.
"This time we will respond to you in America."
The Iranian government also warned that, in the event Iranian soil is bombed, it will retaliate against America's allies in the Middle East by striking Dubai and Haifa, Israel.
Iran's minister of telecommunications Mohammad Javad Azari Jahromi took to Twitter following the missile attack on US targets in Iran.
"Get the hell out of our region!" he tweeted.
US President Donald Trump has been briefed at the White House, but will no longer deliver an address.
White House press secretary Stephanie Grisham said: "The President has been briefed and is monitoring the situation closely and consulting with his national security team."
Top national security officials — including Secretary of State Mike Pompeo and Defense Secretary Mark Esper — have departed the White House this afternoon NZT.
Vice-President Mike Pence also departed.
Trump later tweeted, saying all is well, and he will be making a statement tomorrow.
Meanwhile Iran's Foreign Minister Mohammad Javad Zarif has tweeted that Iran does "not seek escalation or war, but will defend ourselves against any aggression".
Iran has taken responsibility for a series of rockets that were fired at an Iraqi base hosting American soldiers.
Iranian state TV says Tehran launched "tens" of surface-to-surface missiles at Iraq's Ain Assad and Erbil air bases housing US troops at 1.30am local time. It is understood there are very few, if any, casualties.
A US official said 15 missiles were fired. Ten struck the Ain al-Assad air base in Iraq's western Anbar province. One struck a base in Erbil in Iraq's semiautonomous Kurdish region. Four missiles failed to hit their targets.
The official says the bases are still being searched for casualties.
These attacks are a big deal, marking the first direct retaliation against the US for Soleimani's death following three days of mourning.
Soleimani's body is today being laid to rest in his hometown of Kerman.
State TV said the operation's name was "Marytr Soleimani", and that the Revolutionary Guard's aerospace division, which controls Iran's missile program, launched the attacks.
The direct attacks on US troops add to growing fears of a war between the US and Iran.
US defence officials confirm attack
The Pentagon has confirmed that Iran launched missiles towards "at least two Iraqi military bases" in Iraq.
"It is clear that these missiles were launched from Iran and targeted at least two Iraqi military bases hosting US military and coalition personnel at Al-Assad and Irbil," Jonathan Hoffman, assistant to the Secretary of Defence for Public Affairs, said in a statement. "We are working on initial battle damage assessments."
He noted the bases have been on high alert "due to indications that the Iranian regime planned to attack our forces and interests in the region."
"As we evaluate the situation and our response, we will take all necessary measures to protect and defend US personnel, partners, and allies in the region."
Ain Assad air base is located in Iraq's western Anbar province. American forces have been stationed there since the 2003 US-led invasion that toppled Iraqi dictator Saddam Hussein.
It later saw American troops stationed there amid the fight against the Islamic State group in Iraq and Syria.
Shortly after the attack, Iran's top nuclear negotiator Saeed Jalili tweeted a lone image of the Iranian flag:
It appears to be in response to a similar tweet by President Donald Trump, who posted a photo of the American flag following Soleimani's assassination.
A senior US military source told Fox News reporter Jennifer Griffin this afternoon: "Under missile attack from Iran. These are either cruise missiles or short range ballistic missiles. All over the country."
"I'm talking to a well placed source in the US military, on the ground in Iraq, and that source tells me that these are multiple tactical ballistic, or cruise missiles that are being fired from Iran at multiple US bases and US targets inside Iraq," Griffin said on the show. "They are under fire right now. I asked if they had any plans to evacuate, remember there are 5000 American troops in Iraq and I was told 'no, we are defending.' So, expect an American response."
"This is something we have been expecting all afternoon, there were situation room meetings as well meetings with top officials here at the Pentagon," she continued. "There was a lot of chatter, we were told intelligence suggesting that the Iranians would strike at US bases in Iraq after the that mourning for Qassem Soleimani ended earlier today.
"But right now, in real time, we can confirm that multiple sites [have been targeted], including Ain Assad Air Base in Western Iraq, where most of the US troops are based."
Trump has been briefed on the attack, with White House press secretary Stephanie Grisham saying he was aware of the reports.
"The President has been briefed and is monitoring the situation closely and consulting with his national security team," she said.
House Speaker Nancy Pelosi has tweeted about the incident, saying "America and the world cannot afford war".