Four US personnel were killed in a suicide attack claimed by ISIS in the northern Syrian city of Manbij on Wednesday, the Pentagon has confirmed.
It said, "two US service members, one Department of Defence (DoD) civilian and one contractor supporting DoD were killed and three service members were injured while conducting a local engagement in Manbij".
Editorial: Attack demonstrates enduring ISIS threat
"Initial reports indicate an explosion caused the casualties, and the incident is under investigation," it added.
It is the largest death toll incurred by US forces in Syria since a global coalition to fight ISIS was formed in 2014.
The UK-based Syrian Observatory for Human Rights said the blast also killed at least five US-backed fighters as well as nine civilians.
The Kurdish-run Hawar news agency said two US-backed local fighters and 13 civilians were killed in the attack.
Amaq – ISIS's media wing – claimed that the attacker used a suicide vest to strike at coalition forces near a restaurant in the flashpoint Kurdish-held city.
It is the first major attack that explicitly targets US forces since US President Donald Trump announced last month that he would be withdrawing troops from the country.
Turkish President Recep Tayyip Erdoğan on Wednesday said he did not believe the attack would impact Mr Trump's decision to withdraw from Syria.
He said the American president won't halt the withdrawal because it would mean ISIS wins.
Meanwhile, American Vice President Mike Pence on Wednesday maintained the view that ISIS had been defeated, despite the attack on coalition forces.
"The caliphate has crumbled and ISIS has been defeated," he told a gathering of US ambassadors in Washington, without referencing the attack that took place only hours earlier.
He also said that Washington was pressing ahead with plans to pull out its troops.
"Thanks to the leadership of this commander-in-chief and the courage and sacrifice of our coalition partners, we're now actually able to hand off the fight against ISIS in Syria to our coalition partners and we are bringing our troops home," he said.
The US-backed Manbij Military Council, which is linked to the Kurdish People's Protection Units (YPG), has controlled Manbij since they seized the city from ISIS in 2016.
The US-led coalition conducts regular patrols inside Manbij and on the outskirts of the city.
Russia's military police recently deployed to areas outside Manbij, where they are conducting patrols in a zone adjacent to rebel-held territory in Aleppo province.
Wednesday's incident was not the first time that forces of the US-led coalition were subjected to attacks in the area, although they have been rare.
In March last year, a roadside bomb killed two coalition personnel, an American and a Briton, and wounded five in Manbij.