The FBI has requested help from the American public to identify a North American militant who has appeared in Isis (now known as the Islamic State) videos in which he appears to execute Syrian soldiers.

In the terror group's video, Flames Of War, the militant speaks in both Arabic and English but with a North American accent.

The FBI has made the appeal on its website where it has posted part of the video and requested information regarding the masked militant.

"We're hoping that someone might recognise this individual and provide us with key pieces of information," said Michael Steinbach, assistant director of the FBI's Counterterrorism Division, in a statement. "No piece of information is too small."

The video shows a number of men, who are alleged to be fighting for the Syrian regime, digging ditches. The masked militant, whose eyes are only on show, says the soldiers - captured near the Syrian city of Raqqa - were "digging their own graves in the very place where they were stationed".

US intelligence officials have been analysing IS' recruitment videos in the hope of identifying those involved in the films. They believe the North American fighter holds influence within the group as he presents himself as an articulate and persuasive figure.

FBI director James Comey told CBS the US agency was monitoring more than ten American citizens who had flocked to Syria to join terrorist organisations.

"These homegrown violent extremists are troubled souls who are seeking meaning in some misguided way, and so they come across the propaganda, and they become radicalised on their own, sort of independent study, and they're also able to equip themselves with training, again, through the Internet," Comey told CBS. "And then engage in jihad after emerging from their basement."

The CIA estimates over 15,000 foreign fighters from more than 80 countries have joined the civil war in Syria, including 2,000 Westerners. It remains unclear how many have joined IS.

Air strikes conducted by a US-led coalition against the terror group have begun to push its fighters back from some parts of the Syrian border town of Kobani after three weeks of advances.

Clashes between Syrian Kurds and IS militants have killed over 400 people in the battle for the city, according to activists.

The Syrian Observatory for Human Rights said both Kurdish and IS fighters had been killed in the clashes.

The monitoring group said it had documented 412 deaths from sources on the ground but the real figure was likely to be double the official toll.