An attack in the northern Syrian town of Manbij has resulted in multiple casualties including U.S. troops, a senior Kurdish security official confirmed to NBC News.
The blast took place at 1:00 p.m. local time after a suicide bomber in civilian clothing approached coalition forces in the center of Manbij, according to the report. Four Americans, including two servicemembers, a Defense Department civilian and a DoD contractor, were killed.
ISIS claimed responsibility in a post via its Amaq news agency but did not produce evidence in support of the claim.
U.S. troops have been stationed in Manbij in support of local partners, the Syrian Democratic Forces, as part of the anti-IS coalition and as a buffer between Kurdish militias within the SDF and Turkish forces, who view the Kurdish fighters as terrorists.
The attack comes less than a month after President Donald Trump's surprise announcement to withdraw all U.S. troops from Syria, on the premise that ISIS had been defeated.
Despite a flood of criticism from lawmakers and security experts, the withdrawal process is now underway, according to the Pentagon, which has not disclosed details on a timeline or withdrawal numbers for "operational security" reasons.
The attack is likely to complicate the U.S. withdrawal, or potentially accelerate it, depending on the response of the American president.
"The existence of IS sleeper cells is a reality in Manbij, Raqqa and other liberated cities and towns," Mutlu Civiroglu, a Kurdish analyst based in Washington, D.C., told CNBC by phone. He described the cells as "very strong," echoing warnings by numerous U.S. officials in previous months that IS in Syria was far from eliminated.
The attack against Kurdish and American forces, the analyst said, was likely an attempt "to speed up the American withdrawal."
Trump has been briefed on the attack in Syria, the White House said.
"Our deepest sympathies and love go out to the families of the brave American heroes who were killed today in Syria," White House press secretary Sarah Huckabee Sanders said in a statement released to the media. "We also pray for the soldiers who were wounded in the attack. Our service members and their families have all sacrificed so much for our country."
The Pentagon did not immediately respond to CNBC's request for comment.
-CNBC's Amanda Macias contributed to this report.