An American fighting for a hardline Islamist group carried out a deadly suicide bombing, US officials said, in the first such case in the war.
The confirmation came amid growing fears over the flood of foreigners into Syria, with no end in sight in the three-year war that has already left 162,000 people dead.
"The American citizen involved in the suicide bombing in Syria is believed to be Moner Mohammad Abu-Salha," State Department spokeswoman Jen Psaki said on Friday.
Abu-Salha is thought to have been behind a truck bombing against regime forces last Sunday in the northern province of Idlib. He went by the alias Abu Hurayra al-Amriki. Abu Hurayra refers to a companion of the prophet Mohammed and al-Amriki means "the American" in Arabic.
Estimates of the number of foreign fighters who have flooded into Syria in the past three years range from between 9,000 to 11,000, with most believed to have come from neighbouring countries. Psaki could not give precise figures of how many Americans may be among them.
But The New York Times said about 100 Americans were believed to have travelled to Syria, mainly to join the rebels fighting to oust President Bashar al-Assad.
There were few other details about the American, who is said to have joined the al-Nusra Front, which was recently outlawed by the US as a terrorist group.
The Times, citing law-enforcement officials, said he was aged in his 20s, of Middle Eastern descent and from Florida. He is believed to have spent two months in a training camp in Aleppo, and was on his second visit to Syria, returning there late last year.
A Syrian fighter from the al-Nusra Front told The Times that Abu-Salha spoke only poor Arabic, but was dedicated to the extremists' cause.
"He was a generous, brave, tough man, always on the front lines in battles," the fighter said.
"When his turn came up" to carry out a suicide bombing, he "was very happy, because he will meet his God after that," the fighter, who called himself Abu Abdulrahman, told The Times.