A British man named as Ali Almanasfi has been killed while fighting in Syria, together with two other foreigners including an American woman, Syrian TV has reported.

The three are believed to have been killed on Wednesday in Idlib province in the north of Syria. Syrian TV broadcast footage of the dead Briton's passport which appeared to identify him as 22-year-old Ali Almanasfi, born in London in June 1990. The Foreign Office could not confirm any details about the Briton and his identity could not immediately be verified.

Syrian TV also identified the dead American woman as Nicole Mansfield, 33, from Michigan. The American woman's family said she had died while apparently fighting with the rebels against government forces. "I'm just devastated," her aunt, Monica Mansfield Speelman, told Reuters. "Evidently, she was fighting with opposition forces." Speelman said the FBI had informed the family on Thursday afternoon.

Rami Abdul-Rahman, of the British-based Syrian Observatory for Human Rights, said the trio had been ambushed in Idlib province, which is largely controlled by Syria's opposition. The Syrian government controls Idlib city. He told the Guardian: "These people got too close to some military bases near Idlib and were killed."

Abdul-Rahman said he did not know the nationality of the third foreigner. He also said it was unclear if the group was affiliated with the moderate Free Syrian Army (FSA) or a more radical opposition Islamist militia such as the al-Qaida-affiliated Jabhat al-Nusra. The three foreigners had almost certainly crossed into Syria from Turkey, either legally or illegally, he added.

The Syrian TV footage showed a black VW Golf car riddled with bullets, in which the foreigners had apparently been travelling. It also said government forces had retrieved several Kalashnikovs. It showed a photo of what appeared to be the dead Briton, with dark hair and a short beard.

Mansfield Speelman told the Detroit Free Press her niece had married an Arab immigrant several years ago, converted to Islam, divorced, and then two or three years ago moved to Dubai.

"I didn't think she would stoop that low to go over there [Syria] and try to harm anybody," Mansfield Speelman said of her niece.