Air strikes in eastern Syria have killed 26 Iraqis from an Iran-backed Shia paramilitary force after a deadly attack on US-led coalition forces in Iraq, a monitoring group has said.
The Syrian Observatory for Human Rights said Popular Mobilisation bases near the border town of Albu Kamal were hit.
It was not clear who carried out the strikes. The coalition did not comment.
But it followed a rocket attack on Camp Taji military base in Iraq that killed one British and two American soldiers.
No group has said it fired the rockets, but a top US commander said they were likely to have been fired by Shia militias in the Popular Mobilisation.
"While we are still investigating the attack, I will note that the Iranian proxy group Kataib Hezbollah is the only group known to have previously conducted an indirect fire attack of this scale against US and coalition forces in Iraq," Central Command chief Gen Kenneth McKenzie told a Senate committee.
UK Prime Minister Boris Johnson described the attack as "deplorable", while US Secretary of State Mike Pompeo said it would not be tolerated, and that those responsible must be held accountable.
The US has accused Iran-backed militias of 13 similar attacks on Iraqi bases hosting coalition forces in the past year.
The killing of an American civilian in one such incident in December triggered a round of violence which ultimately led US President Donald Trump to order the assassination of the top Iranian general Qasem Soleimani and Kataib Hezbollah commander Abu Mahdi al-Muhandis the following month.
What do we know about the air strikes?
Syrian state news agency Sana said unidentified aircraft "launched an aggression" on Wednesday night on the south-eastern outskirts of Albu Kamal, close to the Syria-Iraq border. The attacks caused only material damage, it added.The Syrian Observatory for Human Rights (SOHR) reported that three aircraft targeted Popular Mobilisation camps in the al-Hassian area and the Imam Ali military base,
The UK-based monitoring group said all of those killed in the strikes were Iraqis, and that weapons and ammunition stores were also destroyed.
Iraqi Shia militias and Iranian forces are based in Syria to support forces loyal to President Bashar al-Assad in the country's civil war.
What happened at Camp Taji?
The Iraqi military base, which is about 15km (9 miles) north of the capital Baghdad, hosts foreign troops from the US-led global coalition against the jihadist group.
A coalition statement said that at about 19:35 (16:35 GMT) on Wednesday Camp Taji was hit by approximately 18 Katyusha rockets. Three coalition personnel were killed and 12 others wounded, it added.
Later, Iraqi forces found a rocket-rigged lorry a few kilometres from the base.
Iraqi journalist Ali Al Dulaimy, who filmed the attack from the nearby town of Baji, said he heard screams of panic from American troops inside the camp, and that he saw them rushing to put out fires.The UK Ministry of Defence said the British soldier who was killed served in the Royal Army Medical Corps, adding that their family had been informed.
The two American personnel who died were active-duty troops with the US Army and Air Force, a US military official told the New York Times.
What was the reaction to the attack?
There was no immediate comment from President Trump. But the British prime minister said: "The attack against the Taji military base in Iraq is deplorable."
"Our servicemen and women work tirelessly every day to uphold security and stability in the region - their presence makes us all safer."
Iraqi President Barham Saleh condemned what he called a "terrorist attack" that had targeted "Iraq's security and its security".
He demanded a full investigation and for those responsible to be held to account.
"While offering our condolences to the families, relatives and the states of the victims, we call upon all sides to exercise restraint and remain calm, [and] to enable Iraq's government to fully manage and carry out its security and sovereign duties."
UK Foreign Secretary Dominic Raab said: "I welcome the Iraqi president's call for an immediate investigation to hold perpetrators to account - but we must see action."
Why is Iraq drawn into the US-Iran confrontation?
Tensions between the arch-foes intensified last year, after Iran-linked fighters targeted US military and civilian personnel in a series of rocket attacks. There were also unclaimed air strikes in Iraq targeting militia facilities and Iranian officials.In late December, a rocket attack on an Iraqi military base killed a US civilian contractor. The US blamed the powerful Kataib Hezbollah militia, which is part of the Popular Mobilisation, and carried out air strikes on its bases in Iraq and Syria that left at least 25 fighters dead.
The US embassy in Baghdad was then attacked by crowds of protesters, and President Trump warned Iran it would "pay a very big price".
On 3 January, Mr Trump authorised a drone strike near Baghdad airport that killed Qasem Soleimani - commander of the Islamic Revolution Guard Corps' Quds Force and architect of Iranian policy in the Middle East - and Abu Mahdi al-Muhandis.
At least three US-led coalition soldiers have been killed after multiple rockets hit an Iraqi base housing US and coalition troops on Wednesday, according to a statement by Operation Inherent Resolve.
The statement added that at least 12 more soldiers were injured during the attack, which is "under investigation by the coalition and Iraqi security forces".In a statement released on Thursday, Iraqi President Barham Salih condemned the attack and pledged to investigate and hold accountable those responsible.
A "number of coalition consultants and trainers within the international coalition operating in Iraq" were killed in the attack, the statement read.
An assessment of the incident is under way, a senior official from the administration of US President Donald Trump said.
"We are closely following the situation at Camp Taji," the official said on Wednesday. "We are not going to get ahead of the assessment and investigation, which are ongoing."The rocket attack was the 22nd against US military interests in the country since late October, an Iraqi military commander said.
US Army Colonel Myles Caggins, a US military spokesman in Iraq, said on Twitter more than 15 small rockets hit the base, but provided no further details.
US Secretary of State Mike Pompeo and Britain's Foreign Secretary Dominic Raab vowed to hold "accountable" the people behind the attack.
In a phone conversation late on Wednesday, the pair "underscored that those responsible for the attacks must be held accountable," the US State Department said.
Camp Taji, located just north of Baghdad, has been used as a training base for a number of years. There are as many as 6,000 US troops in Iraq, training and advising Iraqi forces and conducting counterterrorism missions.
Previous rocket attacks targeting US soldiers, diplomats and facilities in Iraq killed one US contractor and an Iraqi soldier. None of the attacks has been claimed, but Washington accuses pro-Iran factions of being responsible.
Two days after the death of an American in rockets fired on an Iraqi military base in Kirkuk at the end of last year, the US army hit five bases in Iraq and Syria used by the pro-Iran Iraqi armed faction Kataib Hezbollah.
Kataib Hezbollah was designated a "foreign terrorist organization" by the US Department of State in 2009.
Tensions rose further between Washington and Tehran after a US drone strike killed the powerful Iranian military commander Qassem Soleimani and Abu Mahdi al-Muhandis, head of Kataib Hezbollah, in Baghdad on January 3.
The assassination brought tensions between the two countries almost to the breaking point.
The US leads an international coalition - comprising dozens of countries and thousands of soldiers - formed in 2014 to confront the armed group ISIL (ISIS).
While ISIL has lost the vast territory it once held in Iraq and Syria, sleeper cells remain capable of carrying out attacks.
The Iraqi parliament voted to expel all foreign soldiers from the country in the wake of the killing of Soleimani, a decision that has still not been acted on by the government.
The outgoing government, which resigned in December in the face of mass protests, has yet to be replaced because of a lack of agreement in Parliament - one of the most divided in Iraq's recent history.
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