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 group 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.
There were also indications that some French citizens may also be interested in taking up arms in Syria.
France's interior minister said members of a suspected terror cell linked to an attack on a kosher grocery appeared to have plans to travel to Syria to join the civil war unfolding there. Police found bomb-making materials as part of the probe of the group, which was described as "extremely dangerous."
A counterterrorism official in France, speaking on condition of anonymity because he was not authorized to speak publicly on the matter, said authorities have noticed a recent trend for the Syrian conflict to be luring French fighters, 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.
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 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 authorized to speak to reporters.
British officials have said fewer than 100 nationals or people with links to Britain are thought to have traveled 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. U.S. National Director of Intelligence James Clapper said in February that those attacks "bore the earmarks" of the jihadists in neighboring Iraq.
A U.N. panel warned last month that the number of foreign fighters in the conflict was growing a development which it said could radicalize 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 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.
Police said the possible link to the July abduction of the journalists was one of several lines of inquiry being pursuied.
Oerlemans, a prominent Dutch photographer who was shot twice during a failed attempt to escape from his captors, told The Associated Press Wednesday that the ages of the two suspects arrested at Heathrow was consistent with the ages of the people who held him hostage shortly after he entered Syria on July 19. But he said there were no women involved in the kidnapping.
"But that doesn't mean it's not one of the guys in the camp," he said of the man arrested at Heathrow.
The suspects were taken to a central London police station and remain in custody.