Police are investigating claims that a promising Cardiff student could be one of the Islamic State (Isis) militants featured in a video showing the murders of US hostage Peter Kassig and a group of Syrian servicemen.

The investigation comes as French prosecutors confirmed that one of the men shown herding the Syrian soldiers to their deaths was a French national.

Briton Nasser Muthana, 20, who gave up dreams of going to medical school in the UK to secretly travel to Syria, has already appeared in an Isis recruitment film.

On Monday, Muthana’s father, Ahmed, told the Guardian that one of the men in the propaganda video showing the killings of aid worker Kassig and the Syrian men could be his son.

He said: “It looks like him. I can’t be certain because of the quality of the picture and you can never be sure. But it could be him.”

Muthana said Nasser and his brother, Aseel, 17 – who is also believed to be in Syria – had betrayed Britain, where they were born and raised, by leaving to fight with Isis.

He said: “I don’t have sons now – that’s how I feel about them. They would not be welcome back home.” Muthana said he had not spoken to his sons since Aseel left Cardiff. “There has been no communication with me at all. I don’t expect to see them ever again.”

The man standing to the right of a jihadi featured in Isis beheading video is alleged to be Nasser Muthana Photograph: AFP/Getty Images

Muthana, who arrived in south Wales from Yemen in the 1950s to live with an uncle after his father died in a plane crash and his mother was killed in a car accident, expressed sympathy for the families of those murdered by Isis. “I am so sorry for them, I have huge sympathy.”

Asked directly if he believed the man in the video was his son, Muthana said: “No.” He added: “I am like any other father. I am trying not to believe it is my boy but it looks like him.

“But I am not going to make excuses for him. He is a grown man and he must face up to what he has done. It looks like Nasser but he has lost a lot of weight. He was big, maybe 18st [114kg] when I last saw him but he looks thinner in the face in the video.”

Muthana added: “What they [Isis] are doing is inhuman, this is not the son I brought up. He has been got at – he has changed.” Muthana said if it was his son in the video he must be “mentally ill”.

The assistant chief constable of South Wales police, Richard Lewis, said: “We are aware of the reports circulating regarding the involvement of Cardiff individuals in a mass execution in Syria.

“Those reports are being investigated and the details have been passed to the Foreign and Commonwealth Office. Our thoughts are with the victims of these abhorrent acts and those committing such acts can expect to face the full extent of international law.

“Cardiff has a strong and peaceful Islamic community, which has long distanced itself from the extremist views reported. All of Cardiff city mosques have joined together and produced advice on what Islam really says, which promotes moderation not extremism.”

The Muthana brothers hit the headlines in June after Nasser was featured in an Isis recruitment video alongside another man from Cardiff, Reyaad Khan, also 20.

At that time Ahmed Muthana, a retired electrical engineer, told the Guardian he felt as if a bomb had hit his home and family. “I was shocked, I was sad, I cried,” he said. “My wife collapsed. It feels as if the ground under my feet has disappeared.”

He continued: “This is my country. I came here aged 13 from Aden, when I was orphaned. It is his [Nasser’s] country. He was born here in the hospital down the road. He has been educated here. He has betrayed Great Britain.”

The French interior minister, Bernard Cazeneuve, said there was a “very strong probability that a French national took part in carrying out these abject crimes” seen on the Isis video. According to Cazeneuve, the Frenchman was most likely to be Maxime Hauchard.

French investigators say that Hauchard received religious training in Mauritania for a few months in 2012, then headed to Syria via the Turkish town of Gaziantep, posing as a humanitarian worker.

He has never sought to conceal his affiliation with Islamist fighters, and has posted photographs on social media of himself carrying weapons. In July he gave an interview to BFMTV in which he described how he became interested in Islam via the internet and said he decided to travel to Syria to help create a caliphate.