A bomb attack claimed by Islamic State has killed US troops in northern Syria, weeks after President Donald Trump said he would pull out all American forces.
A US official who declined to be named said four US troops had been killed and three wounded in Wednesday's blast. Others said only two had been killed.
An Islamic State-affiliated website said the explosion was the work of a suicide bomber.
The US-led coalition fighting the Islamist militant group confirmed that US service members were killed during an explosion while on a routine patrol, and that it was still gathering details.
The attack took place in the town of Manbij, which is controlled by rebels opposed to Syrian President Bashar al-Assad.
It appears to be the deadliest attack on US forces in Syria since they were deployed on the ground there in 2015.
A Pentagon spokeswoman said that previously only two US troops had been killed in action in Syria.
There have been two additional non-combat fatalities.
Last month, Trump made a surprise announcement that he would withdraw all 2000 US troops from Syria after concluding that Islamic State had been defeated there.
The announcement helped trigger the resignation of his defence secretary, Jim Mattis, stunned allies and raised fears of a long-threatened Turkish military offensive against US-backed Kurdish forces in northern Syria.
A witness in the city said the attack had targeted a restaurant where US personnel were meeting members of the local militia that Washington backs there.
The Syrian Observatory for Human Rights monitoring group said 16 people had been killed in all, including two Americans.
A militia source in north Syria also said two US troops had been killed.
Islamic State later put out a statement saying a Syrian fighter had detonated his explosive vest on a foreign patrol in Manbij.
Two witnesses described the blast to Reuters.
"An explosion hit near a restaurant, targeting the Americans, and there were some forces from the Manbij Military Council with them," one said.
The Manbij Military Council militia has controlled the town since US-backed Kurdish-led forces took it from Islamic State in 2016.
It is near areas held by Russian-backed Syrian government forces and by anti-Assad fighters backed by Turkey.
One of the witnesses said there was a "heavy" presence of military aircraft over Manbij following the blast, which took place near a vegetable market.
