Air strikes in eastern Syria have killed 26 Iraqis from an Iran-backed 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 conducted the strikes. The coalition did not comment.
But it followed a rocket attack on the Taji military base in Iraq that killed a British soldier and two Americans.
Another 12 coalition service personnel were injured by the rockets, which were launched from a lorry discovered a few kilometres from the base.
UK Prime Minister Boris Johnson described the killings as "deplorable", while US Secretary of State Mike Pompeo said they would not be tolerated, and that those responsible must be held accountable.
No-one claimed they launched the rockets, but 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.
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, which is believed to have been built by Iran.
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 Islamic State (IS). Their mission is to train and advise Iraqi security forces.
A coalition statement said that at about 19:35 local time (16:35 GMT) on Wednesday approximately 18 107mm Katyusha rockets struck Camp Taji.
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 and had requested a period of privacy before further details were released.
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."
In response, 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."
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