An American fighting for a rebel group in Syria carried out a deadly suicide bombing in the first such case in the war, US officials have said.

"The American citizen involved in the suicide bombing in Syria is believed to be Moner Mohammad Abu-Salha," the US State Department spokeswoman Jen Psaki said in a statement on Friday.

The statement provided no other details about him.

There are already fears over foreigners fighting in Syria, with no end in sight for a three-year war that has left more than 160,000 people dead.

Florida Senator Bill Nelson told reporters in Miami that the American suicide bomber was from Florida.

Opposition forces had identified the man, who carried out a May 25 truck bombing outside a restaurant in the government-held northwestern city of Idlib, as Abu Hurayra al-Amriki. They said he was a US citizen. The name al-Amriki means "the American".

The truck bombing was one of four by suicide bombers who attacked the area in Idlib province during that day. It is unknown how many people died because of Abu-Salha's bombing.

Foreign fighters

Opposition rebels from the al-Nusra Front, which is fighting to overthrow the government of President Bashar al-Assad, said Abu-Salha's truck was filled with 16 tonnes of explosives to destroy Idlib's al-Fanar restaurant, where Syrian troops were said to gather.

Unlike some other foreign fighters in Syria, the American suicide bomber was not known to have posted messages on Twitter or other social media websites, the Reuters news agency said.

Asaad Kanjo, an opposition activist based in the town of Saraqeb in Idlib province, said he heard that Abu-Salha had arrived in Syria a few months ago and tore up his American passport upon arrival.

"From what I heard, I believe he was an American of Arab origin. People said that he spoke Arabic with a foreign accent, and he used to speak classical Arabic,'' Kanjo said.

Earlier in May, FBI director James Comey said dozens of Americans were joining Syria's civil war.