The US government believes the Daesh militant group is likely responsible for Wednesday's attack in northern Syria that killed four Americans, although it has not reached a firm conclusion, two US government sources said on Thursday, Reuters reports.
The sources, speaking on condition of anonymity, said the Pentagon and other US agencies were investigating who carried out the attack in Manbij, Syria.
Officials studying the incident are not dismissing Daesh's claim of responsibility for the blast, which killed two US troops and two civilians working for the US military, and regard it as plausible if not likely, one of the sources said.
The attack occurred nearly a month after President Donald Trump confounded his own national security team with a surprise decision on Dec. 19 to withdraw all 2,000 US troops from Syria, declaring Daesh had been defeated there.
The Manbij attack appeared to be the deadliest on US forces in Syria since they deployed on the ground there in 2015 and it took place in a town controlled by a militia allied to US-backed Kurdish forces.
If Daesh carried out the attack, that would undercut assertions, including by US Vice President Mike Pence several hours after the blast on Wednesday, that the militant group has been defeated.
Experts do not believe Daesh has been beaten despite its having lost almost all of the territory it held in 2014 and 2015 after seizing parts of Syria and Iraq and declaring a "caliphate."
While the group's footprint has shrunk, experts believe it is far from a spent force and can still conduct guerilla-style attacks. A Daesh statement on Wednesday said a Syrian suicide bomber had detonated his explosive vest in Manbij.
Trump's Dec. 19 announcement was one of the reasons his former defence secretary, Jim Mattis, resigned. It stunned allies and raised fears of a long-threatened Turkish military offensive against US-backed Kurdish forces in northern Syria.
How and when US forces leave has deepened uncertainty in northern Syria, with Turkey and Syrian President Bashar al-Assad ready to fill the vacuum.
The US-backed YPG militia that is allied to the fighters holding Manbij last month invited Assad into the area around the town to forestall a potential Turkish assault. Syrian army troops entered the area soon after.
The YPG-led Syrian Democratic Forces vowed on Thursday to ramp up attacks on Daesh remnants.