BEIRUT (AFP) - Several foreign militants who joined the Islamic State (IS) in Syria but then tried to desert have faced arrest and execution at the hands of the radical group, activists said Friday.

Those who have tried to flee are usually young and mostly non-Arabs who regretted joining IS once they saw the reality on the ground, several activists told AFP via the Internet.

The reports came a day after the IS arrested a 17-year-old militant in the group's self-proclaimed capital of Raqa in northern Syria, according to the Syrian Observatory for Human Rights.

"He was probably Russian or Chechen," said Observatory director Rami Abdel Rahman.

The young man was arrested at a call centre in Raqa, after contacting his family.

"His (militant) friends attacked him and beat him up," said the Observatory, adding that he was then taken to an unknown location.

An IS translator understood the teenager had been telling his family "about how he wanted to go home", the Observatory said.

Activists in Raqa said the case was by no means isolated, but that the dangers faced by would-be defectors kept IS ranks secure for the most part.

"Fleeing is extremely difficult," said Furat al-Wafaa, adding that those who get caught are "slaughtered, of course".

The IS first emerged in the Syrian conflict in April 2013.

It has committed some of the war's worst abuses, including a string of highly-publicised beheadings of Western civilian hostages, as well as executions of Syrian and Iraqi soldiers.

But its reign of terror makes it difficult to assess how many members try to escape, said another activist, Nael Mustafa.

They are "in fact very few in number" and mostly from Western Europe, where they could face prosecution or close surveillance on return, he said.

"We are only talking about individual cases."

Mustafa told AFP of a 19-year-old German militant from Hamburg who had regretted his decision to join IS.

He took a holiday and tried to flee, but the IS cut short his vacation and sent him to a frontline, said Mustafa.

"The next day, I heard that... he died there. Some people said he was shot in the back."

Mustafa reported a small wave of desertions after US-led air strikes were launched against IS in September, but the group had "quickly brought the situation under control."

He said an IS militant's pledge of allegiance, unlike with other rebel groups in Syria, was "considered permanent, like a new birth".