LONDON (REUTERS) - A man said on Monday he believed his son, a British medical student, was among a squad of ISIS extremists filmed beheading Syrian soldiers in the desert in a video posted online on Sunday.

Ahmed Muthana told the Daily Mail newspaper his 20-year-old son Nasser Muthana appeared to be among a group of 16 militants who were seen on a video, which also showed the decapitated head of American hostage Peter Kassig, an aid worker.

"I cannot be certain, but it looks like my son," said Ahmed Muthana, who lives in the Welsh city Cardiff. "He must fear Allah now for killing people. How can he expect to face Allah if he is killing human beings?"

The announcement of Kassig's death, the fifth such killing of a Western captive by the Islamic State in Iraq and Syria (ISIS), formed part of a 15-minute video in which ISIS showed the beheadings of at least 14 men it said were pilots and officers loyal to Syrian President Bashar al-Assad.

French Interior Minister Bernard Cazeneuve said there was a strong likelihood that a 22-year-old French citizen also appeared in the beheading video.

The British Foreign office declined to comment.

ISIS has previously released videos of the beheading of two American and two British men which feature a masked, black-clad militant brandishing a knife and speaking with an English accent, who has been dubbed "Jihadi John" by British media.

Sunday's video showed most of the killers unmasked and the Daily Mail said the man who appeared to be Nasser Muthana was standing alongside Jihadi John.

Nasser Muthana had already appeared in a online video released in June urging Muslims to join ISIS. His younger brother Aseel had also travelled to Syria after both were radicalised in Cardiff, his father told the BBC in June. Nasser Muthana's school friend Reyaad Khan also appeared in the online footage in June, various media reports said.