British police say two men have been arrested at Heathrow Airport on suspicion of participating in terrorist activities in Syria.

Police say the men — both 21 and from Birmingham, central England — are believed to have traveled to Syria in May of last year. Police say neither poses any imminent threat to the public.

West Midlands Police said the men were detained Monday afternoon after they arrived in the U.K. on a flight from Istanbul.

European officials are concerned about domestic threats posed by fighters returning from Syria. British intelligence officials say they have seen "low hundreds of people" from Britain go to Syria to fight.

A handful of people have been arrested in the U.K. on suspicion of attending terrorism training camps in Syria.