Jihadism expert says Sunni Muslims from the UK do everything from suicide bombings to executions in Syria and Iraq conflicts

British extremists are among the "most vicious and vociferous fighters" in Islamic State (Isis) ranks in Syria and Iraq, a jihadism expert has said.

Sunni Muslims from the UK are taking part in the conflicts "in every way", according to Shiraz Maher from King's College London's International Centre for the Study of Radicalisation.

That includes acting as suicide bombers and executioners, he told BBC Radio 4's Today programme on Wednesday after a video purporting to show a US journalist being beheaded by an Isis fighter with a British accent was placed online.

The black-clothed and masked man, who spoke with traces of a London accent, threatened bloodshed in the gruesome video, accusing America of plotting against Muslims and interfering in their affairs before taking a knife to his victim's throat.

If it is confirmed that he is British he will be seen as the most extreme example yet of a fighter travelling from the UK to take part in brutal violence as part of the militant Isis movement.

Maher, a senior research fellow at ICSR, told Today: "Unfortunately the British participation in the conflicts now raging in both Syria and Iraq has been one of full participation, one that has seen them at the front lines, taking part in the conflict in every way.

"So we have seen British fighters out there operating as suicide bombers, we have seen them operating as executioners.

"Unfortunately they are amongst some of the most vicious and vociferous fighters who are out there. That is unfortunately just a part of their radicalisation."

Maher warned that the self-proclaimed caliphate was likely to carry out more atrocities because of western efforts to help people in the region.

"Unfortunately, the way the Foley video is framed, it makes it very clear now that IS (Isis) will react against any western involvement or intervention into the conflicts either in Syria or Iraq and that, of course, given that we are now helping minorities in Iraq – the Yazidis, the Kurds, for example – they regard that not just as an assault on them but they regard that ultimately as a declaration of war against Islam itself.

"And therefore, that is the sort of narrative we have, of course, heard from al-Qaida in the past. That will license them to attack targets and individuals and western interests as they see fit."

Maher said British and other Sunni Muslims initially went to Syria because of an "existential threat" to their faith from Shia Muslims, and their presence has since swelled Isis ranks and allowed it to increase its territory and influence.

In June, two men from Cardiff and a man from Aberdeen featured in an online recruitment video urging western Muslims to join the fighting.

Nasser Muthana, 20, Reyaad Khan, 20, and Abdul Raqib Amin, 26, appeared in the video and are thought to be among more than 400 Britons fighting in Syria and Iraq.

Muthana's father Ahmed, whose other son, Aseel, is also in Syria, spoke out last month as part of a government-backed nationwide campaign encouraging families who suspect their children or siblings are considering going abroad to join militants to speak out. Muthana said his son had wanted to be a GP, but chose "to go with these wrong people".

"I think, 'Am I going to see him alive again?' Maybe we won't even see the coffin – we'll just see on the news they're dead," he said.

Another man, calling himself Abu Osama – whose accent suggests he comes from the north of England – told the BBC last month that he had been fighting in Syria for the establishment of a caliphate.

He referred to Britain as "pure evil", and said he would only return "to raise the black flag of Islam over Downing Street, over Buckingham Palace, over Tower Bridge and over Big Ben".

Manchester twin sisters Salma and Zahra Halane are thought have travelled to Syria in June, possibly to join an older brother who had become a fighter there.

They were described as "deeply religious" but reportedly ignored their family's pleas for them to come home and told them they have no intention of returning.

More than 100 imams have called on British Muslims not to travel to Syria and Iraq, writing an open letter urging local communities "to continue the generous and tireless effort to support all of those affected by the crisis in Syria and unfolding events in Iraq", but to do so "from the UK in a safe and responsible way".