A British medical student and a Frenchman who went to Syria last year are believed to have appeared in a video showing a squad of Islamic State jihadists beheading Syrian soldiers and displaying the severed head of American aid worker Peter Kassig.
Ahmed Muthana told Britain's Daily Mail newspaper his 20-year-old son, Nasser Muthana, appeared to be among the group of 16 jihadists seen in the video.
"I cannot be certain, but it looks like my son," said Ahmed Muthana, who lives in the Welsh capital city of Cardiff.
France's interior minister said analysis by the DGSI security service suggested that one of the men shown herding prisoners to the execution site was Maxime Hauchard, a Frenchman from the northern Eure region who left for Syria in August 2013.
"This analysis suggests with a very high probability that a French citizen could have directly participated in carrying out these abject acts," Bernard Cazeneuve told journalists.
French Judges last year opened a preliminary investigation against Hauchard on suspicion that he was conspiring to commit terrorist acts, the charge commonly levied against citizens who have fought with Islamist militants.
Hauchard was interviewed by French television in the summer saying that his goal in joining Islamic State was to become a martyr.
British Prime Minister David Cameron will chair a meeting of the government's emergency response committee, Cobra, in the next 36 hours to receive briefing from intelligence and security officials in light of the latest video, his spokesman said.
Britain's security threat level was raised to its second-highest in August due to the risks posed by Islamic State fighters returning from Iraq and Syria.
The announcement of aid worker Kassig's death, the fifth such killing of a Western captive by Islamic State, formed part of the video which showed the beheadings of at least 14 men the group said were Syrian military pilots and officers.
Islamic State, which is fighting in Iraq and Syria, includes thousands of foreign combatants and has become a magnet for jihadis from Europe and North America.
IS has 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. Muthana appeared in a video in June urging Muslims to join IS.