At least two airbases housing US troops in Iraq have been hit by more than a dozen ballistic missiles, according to the US Department of Defence.
Iranian state TV says the attack is a retaliation after the country’s top commander Qasem Soleimani was killed in a drone strike in Baghdad, on the orders of US President Donald Trump.
The Pentagon says at least two sites were attacked, in Irbil and Al Asad.
It is unclear if there have been any casualties.
“We are aware of the reports of attacks on US facilities in Iraq. The president has been briefed and is monitoring the situation closely and consulting with his national security team,” White House spokeswoman Stephanie Grisham said in a statement.
Iran’s Revolutionary Guard said the attack was in retaliation for the death of Soleimani on Friday.
“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,” it said via a statement carried by Iran’s state-run IRNA news agency.
Iranian Foreign Minister Javad Zarif later issued a statement on Twitter, claiming the attack was self-defence and denied seeking to escalate the situation into war.
President Trump tweeted shortly afterwards, insisting “all is well”, while adding that they had not yet assessed possible casualties.
Earlier in the day, President Trump said a US withdrawal of troops from Iraq would be the worst thing for the country.
His comments came in the wake of a letter, which the US military said had been sent in error, to Iraq’s prime minister, apparently agreeing to a request by Iraqi MPs to pull troops out.
The US has around 5,000 troops in Iraq.
The UK foreign office told the BBC: “We are urgently working to establish the facts on the ground. Our first priority is the security of British personnel.”
The UK has put the Royal Navy and military helicopters on standby amid rising tensions in the Middle East, Defence Secretary Ben Wallace said earlier.
How did we get here?
The assassination of Soleimani on January 3 was a major escalation in already deteriorating relations between Iran and the US.
The general – who controlled Iran’s proxy forces across the Middle East – was regarded as a terrorist by the US government, which says he was responsible for the deaths of hundreds of American troops and was plotting “imminent” attacks.
Iran vowed “severe revenge” for his death.
Mr Trump, meanwhile, warned the US would respond in the event of retaliation “perhaps in a disproportionate manner”.
“He was a monster. And he’s no longer a monster. He’s dead,” Mr Trump said, defending his decision. “He was planning a big attack, a bad attack for us. I don’t think anyone can complain about it.”
Millions of Iranians turned out for the commander’s funeral, with mourners chanting “death to America” and “death to Trump”.
A stampede at the burial in Soleimani’s hometown Kerman killed 50 people and injured 200 more on Tuesday.
After the event resumed, top Iranian officials renewed their threats of revenge.
“The martyr Qasem Soleimani is more powerful… now that he is dead,” the Revolutionary Guards’ top general, Maj Gen Hossein Salami, told the crowds.
How does Iraq fit into this?
Iran supports a variety of Shia militia groups in neighbouring Iraq. On Friday, Soleimani had just arrived at Baghdad airport and was travelling in a convoy alongside officials from such groups when their cars were hit by several US missiles.
Iraq now finds itself in a difficult position as an ally both of Iran and of the US. Thousands of US troops remain in the country to assist in the broader struggle against the Sunni Muslim Islamic State (IS) group but Iraq’s government insists the US has acted beyond the terms of this agreement.
Prime Minister Adel Abdul Mahdi labelled the missile strike that killed Soleimani as a “brazen violation of Iraq’s sovereignty and a blatant attack on the nation’s dignity”.