"Exercise heightened caution around areas of vehicle movement, including buses, in the Las Ramblas area of Barcelona during Christmas and New Year's. Terrorists may attack with little or no warning, targeting tourist locations, transportation hubs, and other public areas," the State Department's Bureau of Consular Affairs said in a tweet on Sunday.
Asked about the warning during an interview with Rac1 radio on Monday, Catalonia's regional interior minister Miquel Buch said police "were working on this threat".
"We take seriously all threats, all of them are investigated," he added, without giving further details.
Top-selling Spanish newspaper El Pais, citing anonymous police sources, said the authorities were looking for a 30-year-old Moroccan man with a licence to drive buses.
Barcelona-based daily El Periodico de Catalunya said Catalan regional police had circulated an internal note that the man could try to drive a bus into crowds in Barcelona.
Contacted by AFP, a spokesman for the regional police force in Catalonia did not confirm police were seeking a Moroccan man but said a "temporary reinforcement of security measures in areas of high concentration of people" was in place.
Spain has kept its terrorist alert unchanged at the second-highest level despite the US State Department warning, although security measures were boosted in December for the Christmas holiday period, a Spanish interior ministry spokesman said.
Several hours later five of his accomplices mowed down pedestrians on the promenade of the seaside resort of Cambrils south of Barcelona before stabbing a woman to death.
The Islamic State group claimed responsibility for the attack, Spain's deadliest in over a decade.