A British citizen fighting with an extremist rebel group in Syria has issued a call for others to join the country’s civil war.

In a video posted online, a masked man speaking in a London accent says that “the doors of jihad are still open,” and claims that he has helped three other Brits make the journey to Syria.

The man is believed to be a member of the Islamic State of Iraq and the Levant (Isis) – an extremist Islamist rebel group formerly affiliated with al-Qa’ida and similar in ideology. He has appeared in several other videos uploaded by a group called Rayat al-Tawheed, which claims to be the distributor of English-language materials for Isis.

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The plea from the rebel fighter comes as UK authorities are becoming increasingly concerned at the number of British citizens travelling to Syria to join extremist groups in the three year old civil war. British and other non-Syrians travelling to Syria tend to gravitate towards al-Qa’ida-type groups that espouse global jihad.

The total number of British participants in the conflict is estimated to be in the “hundreds”, with as many as 20 thought to have died in the fighting. Charles Farr, the Home Office’s terror chief, warned recently that Brits travelling to Syria represented the “the biggest challenge” to the security services since the 2001 Twin Tower attacks.

In a previous video, the same man spoke of the difficulties of fighting in Syria, saying “you don't just come here, put on a tactical vest, grab a Kalashnikov and get a big beard.” In the video uploaded yesterday he takes a different approach, and actively encourages others to join him.

“We have managed to bring in three or four beloved brothers of Islam. You can see for yourself,” he says, while sitting atop a heavy machine. The camera then turns to another man, wearing a balaclava, who also speaks with a British accent.

“We send you the greetings of Islam from the blessed lands of al-Sham. By the grace of Allah [..] he has enabled us to enter the blessed lands. The lands which the kuffar back home always tell you that if you plan to reach there will be a consequence for you.”

Charles Lister, visiting fellow at the Brooking Doha Centre and an expert on Syrian rebel groups, said the latest video “serves to underline that despite the on-going infighting in northern and eastern Syria, there is still a desire among some British nationals to join the Syrian jihad.”

In recent months, a backlash against extremists in Syria has seen more moderate fighters force Isis into retreat across the north of the country – taking back territory that was held by the group. Isis, which has been blamed for some of the worst atrocities carried out by the rebels, still holds territory across much of north and eastern Syria.

“With Isis and the issue of foreign fighters under particular pressure in Syria right now, not to mention the tight restrictions within UK border security, it's quite telling that a video like this one would be released emphasising so strongly that travelling to Syria is still possible and worthwhile,” Mr Lister added.

Thousands of foreign fighters are thought to have travelled to Syria since the beginning of the civil war in 2011, with many of them crossing Syria’s porous border with Turkey. Turkish authorities have recently sought to exert more control over its border as extremist groups have grown in strength.

As Isis and other jihadist groups have come under greater pressure from other rebels, many foreign fighters have fled Syria and returned to their country of origin. Senior security officials have said that around 250 British “extremist tourists” have returned home, and are now suspected of wanting to carry out attacks here. Scotland Yard revealed earlier this year that Syria-related terrorism arrests are soaring with 16 so far this year, compared with 24 for all of 2013.