U.S. forces were killed in a blast in northern Syria on January 16, the U.S.-led coalition against the Islamic State (IS) group has said in a tweet.
"U.S. service members were killed during an explosion while conducting a routine patrol in Syria today. We are still gathering information and will share additional details at a later time," a coalition spokesman said.
Earlier, Reuters quoted an unnamed U.S. official as saying that four U.S. soldiers were killed and three wounded in the blast in the northern Syrian town of Manbij.
A Syrian war monitoring group said the blast near a patrol of the U.S.-led coalition in Manbij killed and wounded more than a dozen people.
The Britain-based Syrian Observatory for Human Rights said 16 people were killed, including at least five U.S.-backed Syrian fighters. It added that nine civilians and others were wounded in the blast.
The attack came days after the United States began the process of withdrawing from Syria, pulling out equipment from the northeast into neighboring Iraq.
There was no immediate comment from the U.S.-led coalition on whether there were casualties among coalition forces.
But Reuters quoted an unnamed U.S. official as saying that four American soldiers were killed and three wounded in the blast.
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.
The Observatory's chief Rami Abdurahman said the blast was caused by a suicide bomber but didn't immediately offer any further details.