A US citizen carried out a suicide bombing against Syrian troops on Sunday, the US state department says.
The rebel al-Nusra Front said the man conducted the bombing on their behalf. It was one of four attacks carried out in the northern city of Idlib that day.
It is thought to be the first suicide attack by a US citizen in the conflict.
More than 100,000 people have been killed in the battle between forces loyal to Syrian President Bashar al-Assad and those opposed to his rule.
"I can confirm that this individual was a US citizen involved in a suicide bombing in Syria," state department spokeswoman Jen Psaki said.
The man fought under the name Abu Hurayra al-Amriki, which translates as "the American", she said.
The department was "concerned about the flow of foreign fighters in and out of Syria", Ms Psaki added.Al-Nusra Front at a glance
- Aims to establish Islamist state in Syria
- Leader: Abu Mohammad al-Julani
- 5,000 official members (approx.), supported by thousands of others
- Apparently has members inside government and military
- Big guerrilla attacks on rural government targets; lower level urban attacks
- Uses car bombings, suicide attacks, targets media facilities and personalities
- Policy of silence
Source: Quilliam Foundation
The al-Nusra Front, a militant Islamist group that has pledged allegiance to al-Qaeda, said the man used a truck carrying explosives to conduct the attack.
The Syrian Observatory for Human Rights said dozens of soldiers were killed by the series of bombings on Sunday, AFP news agency reported.
Syria's internal conflict, which began in 2011, has destroyed whole neighbourhoods and forced nearly three million people to flee the country.