Two sets of British brothers who are cousins 'travelled to Syria to fight alongside Isis'

The men are said to make the largest number of Isis fighters in a British family

A member of Ussud Al-Anbar, a group that is affiliated to Isis, holding up the black and white flag in Iraq’s Anbar province (AFP/YouTube)

Four members of the same family are suspected of having flown to Syria to fight alongside the self-proclaimed Islamic State.

Two brothers aged 17 and 20, from Camden in north London, are believed to have travelled with their cousins via Milan to Istanbul before they all crossed the border into Syria, The Sunday Times reports.

The boys from London, who have not been named, were reported missing by their parents when they noticed that they had disappeared sometime in September.

Their cousins, also brothers and are from Wednesbury in the Midlands, have been identified as Mejanul Islam, 22, and Kamran Islam, 19, and their parents told the Sunday Times that the young men had told them that they were going to London to visit their extended family.

Their father Saydul Islam, 43, told the newspaper that there was no concrete knowledge as to where they went or when they are expected to return.

"We've had no news at all," he had said. "I don't know where they've gone. We're very worried."

Counter-terrorist police have subsequently carried out searches at two properties in London and at the family home in Wednesbury and Scotland Yard are currently investigating the case.

The Home Office state that they do not comment on individual cases - however they advise against travelling to Syria - and that they deal with citizenship matters for people suspected of fighting alongside Isis on a case by case basis.

It is believed that the two sets of siblings would constitute the largest number of members from the same family that have left Britain to allegedly fight with Isis, however four brothers from Australia were reported in November to have travelled to Syria.

The mother of the siblings - aged 17, 23, 25 and 28 - said they claimed to have gone on holiday to Thailand and the family had no idea that they had plans to actually travel to Turkey to potentially cross the border into Syria, The Guardian reported.

Community leader Dr Jamal Rifi, in western Sydney, said the family had no inkling that the boys were at risk of joining Isis.

"The boys are simple boys," he had said. "They are religiously involved but not hardline or radical."

Two Austrian sisters, known only as Viktoria, 16, and Violetta, 17, are believed to have joined Isis after they went missing from Vienna on 1 November with just their passports.

More than 500 British Muslims are said to have travelled to Syria and Iraq to fight with Isis.