At least one American national was killed in the twin attacks in Spain, with US authorities working to verify if other Americans were caught up in the violence, Secretary of State Rex Tillerson said Friday.

Tillerson said he could confirm the death of "one American citizen," and added that Washington was "still confirming the death or injuries" of other US nationals.

"We offer our thoughts and prayers to their families," Tillerson said.

Spanish police hunted Friday for the driver who plowed a van into crowds of pedestrians on a busy avenue in Barcelona, just hours before a second such assault hit a nearby resort. The twin attacks left 14 dead and over 100 injured.