Trump is on a working vacation at his private golf club in New Jersey. He was being briefed on developments in Spain by his chief of staff, John Kelly, the White House said.

First lady Melania Trump tweeted her "thoughts and prayers'' to Barcelona before the president condemned the attack.

At least 13 people were killed and scores were wounded, some seriously, after a van was driven onto a sidewalk and down a pedestrian zone in Barcelona's historic Las Ramblas district. The Islamic State group claimed responsibility.

Secretary of State Rex Tillerson offered condolences to victims of the attack during an appearance before journalists at the State Department shortly after it happened. He said the incident "has the hallmarks of what appears to be yet another terrorist attack.''

Tillerson said US diplomats in Spain are assisting Americans there, and he asked those who are safe to notify friends and families. Tillerson said the United States will never relent in tracking down terrorist suspects and holding them to account for their actions.

"Terrorists around the world should know that the United States and our allies are resolved to find you and bring you to justice,'' Tillerson said.

Spanish authorities said there have been two arrests in connection with the attack. (AP)