Oerlemans, a prominent Dutch photographer who was shot twice during a failed escape attempt, told reporters on Wednesday that the ages of the two suspects was consistent with the ages of the people who held him hostage shortly after he entered Syria on July 19.

“There wasn’t any woman anywhere, so that surprised me,” he said of the Heathrow arrests. “But that doesn’t mean it’s not one of the guys in the camp. They were all 20-somethings so it might just well be the kidnappers.”

Police would not comment on British press reports that one of the suspects may be a British National Health Service doctor believed to have been involved with a terror cell inside Syria. Cantlie had told the British press earlier that one of his captors claimed to be a British medic who said he had taken a sabbatical from his work so that he could treat badly injured fighters in Syria.

Most of those fighting the regime of President Bashar al-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 becoming involved in the Syrian conflict.

Syrian rebels concerned about alienating Western supporters have downplayed the newcomers’ impact on the struggle to dislodge Assad.

“It’s exaggerated, the role and the numbers,” said George Sabra, spokesman for the Syrian National Council. He blamed in part the international community for using the issue as a way to back out of commitments to help the Syrians opposing Assad.

He said the foreign fighters do not present long-term problems.

“They say they’ve come to help the Syrian people and they’ll return home again,” he said. “We’re not worried because they’re an external phenomenon and they’ll remain one.”

European intelligence officials said the issue of foreign fighters joining the Syrian conflict is complex. There is evidence that foreigners have joined the fight against the Assad regime, but the number of foreign extremists joining the fight remains unclear.

“We know there have been British, French and other Western nationals joining this fight in Syria but we’re not seeing enough evidence to indicate there is a groundswell of foreign extremists on the battlefield. On the contrary, it seems that some of these fringe groups are waiting for an opportunity to come after the battle is over,” said a European security official who spoke on condition of anonymity because he was not authorised to speak to reporters.

British officials have said fewer than 100 nationals or people with links to Britain are thought to have travelled to Syria to fight against Assad’s regime.

Officials have said unlike Libya, it’s harder to track individuals getting into the conflict area since there are multiple entry points.

Although officials aren’t worried about an immediate threat to the UK, officials have warned that any skills learned on the Syrian battlefront could present potential threats to Britain and beyond.

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.

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.

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 1,200-1,500 foreign fighters across Syria.

A counterterrorism official in France, speaking on condition of anonymity because he was not authorised to speak publicly on the matter, said authorities have noticed a recent trend for the Syrian conflict to be luring French, and the phenomenon has been increasing in the last few months. He would not specify how many people that might involve in France, or whether anyone had been detained.

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.