Barcelona was on high alert Monday following a warning from the US about the risk of a terrorist attack in the Spanish city over the Christmas holiday.

“Exercise heightened caution around areas of vehicle movement, including buses, in the Las Ramblas area of Barcelona during Christmas and New Years,” the US State Department’s Bureau of Consular Affairs tweeted.

“Terrorists may attack with little or no warning, targeting tourist locations, transportation hubs, and other public areas.”

Catalan police, the Mossos d’Esquadra, wouldn’t comment directly on the alleged threat, saying only that security was heightened “as part of a previously arranged anti-terrorist vigilance for the Christmas period.”

But local paper El Pais reported that the cops, based on tips they’ve received, are looking to question a 30-year-old Moroccan man with a bus driver’s license. The man, identified as B.L., has a record for insulting a member of the Civil Guard at Malaga airport in 2006, the paper said, citing sources.

Barcelona’s downtown boulevard, La Rambla, popular with tourists, was targeted last year by an ISIS militant who rammed a rental van into pedestrians, killing 14.

Police currently are carrying out checks on bus and minibus drivers in the center of Barcelona and monitoring car rental companies, El Pais reported.

With Post wires