WASHINGTON (Reuters) - The U.S. government believes the Islamic State 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 U.S. government sources said on Thursday.

The sources, speaking on condition of anonymity, said the Pentagon and other U.S. agencies were investigating who carried out the attack in Manbij, Syria.

Officials studying the incident are not dismissing Islamic State’s claim of responsibility for the blast, which killed two U.S. troops and two civilians working for the U.S. 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 U.S. troops from Syria, declaring Islamic State had been defeated there.

The Manbij attack appeared to be the deadliest on U.S. 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 U.S.-backed Kurdish forces.

If Islamic State carried out the attack, that would undercut assertions, including by U.S. Vice President Mike Pence several hours after the blast on Wednesday, that the militant group has been defeated.

Experts do not believe Islamic State 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. An Islamic State 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 defense secretary, Jim Mattis, resigned. It stunned allies and raised fears of a long-threatened Turkish military offensive against U.S.-backed Kurdish forces in northern Syria.

How and when U.S. forces leave has deepened uncertainty in northern Syria, with Turkey and Syrian President Bashar al-Assad ready to fill the vacuum.

The U.S.-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 Islamic State remnants.