At least four U.S. soldiers were killed and three wounded in a blast on Wednesday in the Syrian town of Manbij, a U.S. official said. A war monitor said at least 16 had been killed in total. White House says U.S. President Donald Trump has been briefed on the situation.

A news site affiliated with Islamic State earlier issued a statement saying an attacker with a suicide vest had targeted a patrol of the U.S.-led coalition operating in Manbij.

Reuters could not independently verify a report by the UK-based Syrian Observatory for Human Rights that 16 people had been killed in the attack including two U.S. soldiers. The coalition could not be immediately reached for comment.

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Open gallery view Areas of control in Syria Credit: AP graphics

Last month, Trump made a surprise announcement that he would withdraw all 2,000 U.S. troops from Syria after concluding Islamic State had been defeated there. The announcement rattled allies in the region and top U.S. officials, including Secretary of Defense Jim Mattis who quit.

Two witnesses described Wednesday’s blast to Reuters. The U.S.-backed, Kurdish-led Manbij Military Council militia that controls the town said there was preliminary information that people had been injured in Wednesday’s attack.

Manbij has been held by U.S.-backed fighters allied to the Kurdish YPG militia since they took it from Islamic State in 2016. It is located near areas held by Russian-backed Syrian government forces and by anti-Assad fighters backed by Turkey.

One of the witnesses said there was a “heavy” presence of military aircraft over Manbij following the blast, which took place near a vegetable market.