(CNN) -- Amid a high alert over an alleged al Qaeda plot to attack cities in Europe, France has warned its citizens about travel to the United Kingdom, saying British authorities believe a terror attack is "highly likely." The move follows a United States warning that Americans be careful about traveling in Europe because of the risk of terror attacks. Neither the UK nor France raised threat levels in response to the American warning, but each has now warned its citizens about travel elsewhere on the continent. Europe remains on edge after the warnings, based at least partly on intelligence about a plot obtained from a German-Afghan in U.S. custody in Afghanistan.
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French warning: British authorities believe an attack is likely
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U.S., others have warned about travel in Europe because of terror threats
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U.S. warning based on intelligence from a German-Afghan in U.S. custody
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This week has seen arrests and suspected drone strikes against terror suspects
@placeholder national police said the 12 suspects were seized in the south of France.