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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 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, 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.

Paisley Dodds in London, Elizabeth A. Kennedy in Beirut, Lori Hinnant and Jamey Keaten in Paris and Michael C. Corder in Amsterdam contributed.