BRITISH police are investigating whether a British man and woman arrested on suspicion of supporting terrorism offences in Syria were part of a group that held a veteran war journalist hostage last month.

The abduction of freelance photographer John Cantlie in August highlighted concerns that British Muslims might be slipping into Syria to join extremists. Police in Britain seeking clues in the case searched two east London properties – one day after the pair were arrested at Heathrow Airport after arriving from Egypt.

The investigation is only one line of inquiry, police said while speaking on condition of anonymity in line with force policy. Cantlie had said upon his release that one of his captors spoke with a London accent.

Most of those fighting the regime of President Bashar Assad are believed to be ordinary Syrians and soldiers who have defected, having become fed up with the authoritarian government, analysts say. But increasingly, foreign fighters and those adhering to an extremist Islamist ideology are turning up on the front lines.

The rebels are trying to play down their influence for fear of alienating Western support, but as the civil war grinds on, the influence of these extremists is set to grow.

The Syrian government has always blamed the uprising on foreign terrorists, despite months of peaceful protests by ordinary citizens that only turned violent after repeated attacks by security forces. The transformation of the conflict into an open war has given an opening to the foreign fighters and extremists.

Talk about the role of foreign jihadists in the Syrian civil war began in earnest, however, with the rise in suicide bombings. US National Director of Intelligence James Clapper said in February that those attacks “bore the earmarks” of the jihadists in neighbouring Iraq.

Rebel commanders are quick to dismiss the role of the foreign fighters and religious extremists, describing their numbers as few and their contribution as paltry.

A UN panel warned last month that the number of foreign fighters in the conflict was growing – a development which it said could radicalise the rebellion against Assad’s rule. The Quilliam Foundation, a London-based think tank studying extremism, estimated that there were a total of 1200-1500 foreign fighters across Syria.

Britain’s Foreign Secretary William Hague warned Britons against travelling to Syria to take part in the fight to depose Assad. Hague told the BBC that the government is aware of some Britons joining the pitched battles for control of Syria.

“That’s not something we recommend, and we do not want British people taking part in violent situations anywhere in the world,” he said.

A British police statement said the man and woman were arrested on suspicion of the “commission, preparation or instigation of acts of terrorism.” The statement did not include the suspects’ names or any other identifying information.

The suspects were taken to a central London police station and remain in custody.