Diplomats have quietly opened a new front in efforts to stem the flow of Britons travelling to fight in Syria by spending nearly £200,000 on "social media activity" to deter would-be jihadis from leaving the UK, The Independent on Sunday can reveal.

Fighters for al-Qa'ida-linked groups in Syria's civil war have proved adept at using social networks such as Twitter and Facebook to recruit foreign fighters, with messages and images exhorting young Muslims to join a holy war against Bashar al-Assad's brutal regime.

Anil Khalil Raofi, a 20-year-old student from Manchester who last month became the latest Briton to die in Syria, is known to have left Britain after reading posts by a fellow Briton, Ifthekar Jaman. The 21-year-old from Southsea, Hampshire, had joined the ultra-violent al-Qa'ida splinter group, Islamic State in Iraq and the Levant (Isil), currently embroiled in rebel infighting. Raofi, an engineering student, had begun posting instructions on how other Britons could join him prior to his death.

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Now the Foreign and Commonwealth Office (FCO) aims to counter the pro-jihadi propaganda glorifying the conflict by setting up its own online project to convince British Muslims not to go to Syria.

Documents seen by The IoS show that the FCO was granted £173,000 in urgent funding for "social media activity to deter UK residents from travelling to fight in Syria". The spending, part of a project entitled "FCO deterring Syria foreign fighters activity", had to be approved by a special Treasury unit, the Efficiency and Reform Group, because it fell outside public sector spending limits. But although the budget and aim of the internet campaign can be revealed, the precise methods being used to win the battle for the hearts and minds of young Muslims are unknown.

Last week, The IoS made repeated requests to the FCO to detail which channels it uses in its social media programme and how it uses them. But the department said only that it was "emphasising the genuine risks of travel and the reality of the dangerous situation on the ground".

In the United States, the State Department has started using an official Twitter account with a department crest to target Islamist extremists or potential terrorists online, messaging recipients with the hashtag #thinkagainturnaway.

But Whitehall reticence on whom it messages, and how, suggests that the British project may be posting material or approaching individuals without declaring its origins, for fear of losing credibility with its target audience. Official FCO Twitter and Facebook accounts rarely contain material directly addressing potential jihadis.

Shiraz Maher, a senior fellow at the International Centre for the Study of Radicalisation at King's College London, said: "Around the time of Iraq and Afghanistan, there were attempts to try to get messages online to counter the extremists, which was the old way of thinking. It is likely the Government is refining that approach. Syria has been unprecedented in the way the internet and social media have been harnessed.

"The jihadis themselves are able to communicate directly for the first time about what is going on. They are taking to Twitter and Facebook, often to express arguments between themselves. Even the extremist groups themselves have lost control of the narrative."

Counter-terrorism officials have described the growing numbers of Britons involved in the Syrian conflict as most serious terrorist threat to the United Kingdom since the 9/11 attacks. Scotland Yard's former head of counter-terrorism, Commander Richard Walton, who this weekend was moved from his post following the publication the Ellison Review into the Stephen Lawrence murder, said earlier this year it was "almost inevitable" that some Britons fighting in Syria would seek to carry out attacks in Britain on their return.

While the nature of the FCO campaign remains unclear, British Muslim groups are also taking their own steps to try to reach potential recruits to the Syrian war. One Suffolk-based group, Jimas, last month posted a YouTube video entitled "Should I travel to Syria for Jihad?". The charity's chief executive Abu Muntasir tells viewers: "You will end up prolonging the conflict in Syria. A person of conscience would not want to get voluntarily involved in creating more strife and pain and conflict in any situation or society."

Last month, 41-year-old Abdul Waheed Majeed, a father of three from Crawley, West Sussex, became the first British suicide bomber in Syria when he drove a truck bomb into Aleppo central prison.

With propaganda playing a central role in drawing Britons and other young Muslims in Western countries to Syria, security agencies and police have long monitored jihadi Twitter feeds and other social networks for intelligence on potential threats. But their use as a government publicity tool is new and its effectiveness unknown.

It is estimated that about 350 UK residents have gone to Syria to fight. Sixteen people were held in January alone on suspicion of Syria-related offences, including two 17-year-old girls detained at Heathrow. The figure for the whole of 2013 was 24.