A U.S. official who declined to be named said four U.S. troops had been killed and three wounded in the blast, which an Islamic State-affiliated site said was the work of a suicide bomber. Others said only two had been killed.

The U.S.-led coalition fighting the Islamist militant group said that “U.S. service members were killed during an explosion while conducting a routine patrol”, and that it was still gathering details.

The attack, which took place in the town of Manbij, controlled by rebels opposed to Syrian President Bashar al-Assad, appears to be the deadliest on U.S. forces in Syria since they deployed on the ground there in 2015.