Iran says it is "ready for war" after shooting down a US drone as a series of escalating clashes between Tehran and Washington stoked fears of military conflict.

The simmering tensions in the Middle East have deepened, with the commander of Iran's Revolutionary Guard saying shooting down the drone sent "a clear message" to America.

"We do not have any intention for war with any country, but we are fully ready for war," General Hossein Salami said in a televised address.

Less than a week after the US accused Iran of attacking tankers, the Islamic Revolutionary Guard Corps said its air force brought down an American surveillance drone in a southern coastal region along the Strait of Hormuz. The IRGC said the drone had violated its airspace and crashed into Iranian waters.

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The US military confirmed that an Iranian surface-to-air missile shot down an unmanned drone, but denied the aircraft was in Iranian airspace.

Captain Bill Urban, a US Central Command spokesman, said: "This was an unprovoked attack on a US surveillance asset in international airspace.

"There was no drone over Iranian territory."

He confirmed the drone was a RQ-4A Global Hawk and provides real-time intelligence, surveillance and reconnaissance missions over vast ocean and coastal regions.

The state-run Islamic Republic News Agency said on Thursday the drone was hit when it entered Iranian airspace near the Kouhmobarak district in southern Iran's Hormozgan province.

The US military confirmed a US RQ-4 Global Hawk was shot down by Iran, but denied the drone was in Iran airspace. Photo / AP
The US military confirmed a US RQ-4 Global Hawk was shot down by Iran, but denied the drone was in Iran airspace. Photo / AP

The drone's downing was the latest confrontation in the stand-off between Iran and the US, stemming from Donald Trump's decision a year ago to withdraw America from Iran's nuclear deal with world powers.

Oil prices rose sharply after news of the drone attack, with Brent crude, the international benchmark, up 2.67 per cent at $62.47.

Tensions between the countries have reached fever pitch after the mysterious oil tanker attacks last week. The US blames Iran, which denies it was involved in the attacks.

The US also claimed Iran had fired a missile at another drone that was responding to the attacks on the ships.

US Secretary of State Mike Pompeo has also alleged there are alarming ties between Iran and al-Qaeda. Founded by Osama bin Laden, al-Qaeda was, of course, known as the band of terrorists that masterminded the September 11 attacks, and it's feared the powerful new claim could lead to a war between the USA and Iran.

Many Democrats and Republicans are worried the Trump administration is building a case the White House could use to invoke a war based on laws passed by Congress in 2001 to battle the infamous terror group.

It's understood Pompeo is now trying to convince the US Congress there is a pattern of ties between Iran and al-Qaeda going back to just after the 2001 terror attacks.

If the link is proven, it would legally allow the Trump administration to go to war with Iran.

The key anti-terror laws were passed under George W. Bush, and they allowed the USA to go to war in Afghanistan, where bin Laden found shelter.

They have since been used to justify operations in countries as diverse as Yemen and the Philippines where Al-Qaeda militants were found to be at large.

SERIOUS DOUBTS OVER CLAIMS

US politicians who fear the Trump administration is warmongering have hit out at Pompeo's claims.

"They are looking to bootstrap an argument to allow the President to do what he likes without coming to Congress, and they feel the 2001 authorisation will allow them to go to war with Iran," Democrat Senator Tim Kaine told the New York Times.

It's not the first time Pompeo has made the link between Iran and the terror group, telling a Senate committee in April there is "no doubt" of a connection.

The claims are considered dubious by some because of the major ideological differences between Iran's Shiite clerical regime and Al-Qaeda.

Al-Qaeda is militantly Sunni and was founded on an ultraconservative Islamic doctrine in Saudi Arabia — one of Iran's biggest enemies.

US Secretary of State Mike Pompeo says there's 'no doubt' of a connection between Iran and al-Qaeda. Photo / Washington Post
US Secretary of State Mike Pompeo says there's 'no doubt' of a connection between Iran and al-Qaeda. Photo / Washington Post

Despite this, Iran is believed to have been the longtime base of Hamza bin Laden, the son of Osama.

However, some experts believe Tehran may have been holding him as leverage to prevent attacks against Iran or to put pressure on Saudi Arabia.

Trump has said he does not want a war with Iran, as he has often lashed out at the 2003 invasion of Iraq.

One America News reports Trump has urged his administration officials to "tone down" their talk about Iran, reminding them he is not interested in a military confrontation with Tehran despite the hawkish views of some of his advisers

However, Trump ordered 2500 additional troops to the region in the last month in response to what American officials said was a heightened threat.

WHY ARE TENSIONS SO HIGH?

Tensions have soared in recent weeks, with the United States seeking to end all Iranian oil exports and Washington pointing to Iran as the culprit in a series of attacks on tanker ships.

People walk by a mural which reads 'Down with the USA' in Tehran. Photo / Bloomberg
People walk by a mural which reads 'Down with the USA' in Tehran. Photo / Bloomberg

This week, the US government released new photos to bolster its claim Iran was responsible for the attacks on two oil tankers in the Gulf of Oman on June 13.

It said the photos, taken from a navy helicopter, showed Iranian forces removing an unexploded mine from the side of the Japanese-owned Kokuka Courageous.

Another picture shows a massive hole on the side of the Courageous above the water line that officials say appears to have been caused by another mine.

Pompeo and other top officials are now reaching out to leaders in Asia and Europe to convince them Iran was behind the attacks — leading to a spike in tension with Iran.

The Trump administration says its "maximum pressure" campaign — which included pulling out of Barack Obama's denuclearisation deal still backed by the Europeans — was succeeding, pointing to financial difficulties faced by Iran's militant allies such as Hezbollah and Hamas.

But Brian Hook, the US special representative on Iran, said Trump was serious in his appeals for negotiations with Iran.

"No one should be uncertain about our desire for peace and our readiness to normalise relations should we reach a comprehensive deal," Hook told a House Foreign Affairs subcommittee overnight.

"We have put the possibility of a much brighter future for Iran on the table and we mean it."

Hook said a future US-Iran deal would focus on four areas — Tehran's nuclear program, ballistic missiles, support for extremist groups and detention of US citizens.

Iran's leaders have dismissed dialogue with the Trump administration, questioning its sincerity and pointing to US officials' comments gloating over economic difficulties in the country.

Representative Ted Deutch, the Democrat who heads the House subcommittee on the Middle East, said Trump's policy was incoherent and had triggered an escalation from Iran rather than any desired outcome.

"It appears there is no process in place to reassess the assumptions underlying the administration's policies," he told AFP.

"Rather than force Iran back to the negotiating table, the administration's policy is increasing the chance of miscalculation, which then would bring the United States and Iran closer to a military conflict."

— with wires