Scores of extremists, some newly converted to Islam, have been heading off to the Middle East and MI5 and counter-terrorism police units are using anti-terror powers to disrupt their travel plans.
The civil war is largely seen as a fight between the Free Syrian Army and the repressive regime of Bashar al-Assad.
However, Western governments are increasingly worried that it is being hijacked by jihadist groups inspired by Al Qaeda ideologies.
A small number of British Muslims have already been detained and questioned over concerns they intend to join extremist brigades fighting with the rebels.
One security source warned: ‘There is an increasing number of Muslims going to Syria but they are becoming difficult to monitor because the disguise their ultimate destination by first travelling to France before flying on to Turkey and Jordan.
‘The question is when they return to the UK do they carry on their lives peacefully, happy to have done their bit for jihad, or do they bring jihad back to Britain?’
The source added: ‘Some have the will to carry out attacks and by the time they return they also have capacity to make bombs and fire guns. This is a concern that needs to be closely monitored.’
Unlike the Libyan conflict, the Syria volunteers are not refugees from their home country and counter-terrorism agencies fear that once the war is over, these trained fighters will look for a new jihadist cause.
There are up to 100 British fighters in Syria, and some of Pakistani origin have been identified in the forefront of foreign brigades.
A team of MI6 officers working with GCHQ’s monitoring station in Cyprus is trying to trace the whereabouts of British fighters, including one who said he was a London hospital doctor.
The man was one of a group of jihadists who kidnapped British journalist John Cantlie and Dutchman Jeroen Oerlemans.
The pair were held for a week before being rescued and said many of the fighters appeared to be British Pakistanis with London accents.
Cantlie said the man who treated him for wounds claimed to be an NHS doctor on sabbatical from A&E at a South London hospital.
He added: ‘The doctor was one of the few guys who was half decent to us. But this was a group of kids on “jihad holidays” and he was an integral part.’