Australian Prime Minister Malcolm Turnbull has asked people to pray for a young Australian boy who is missing after a vehicle attack on a popular Barcelona promenade that killed 13 people.

Turnbull said Saturday that the boy's mother was badly injured and is in a hospital following Friday's attack at the Las Ramblas promenade. Turnbull says the boy's family is searching for him. He did not release the boy's name.

Turnbull says: "All of us as parents know the anguish his father is going through, and his whole family is going through, as they rush to seek to find him in Barcelona."

Australian Foreign Minister Julie Bishop says four Australians were injured in the attack.

Police in Catalonia say they have searched two buses in their hunt for violent extremists believed responsible for a pair of deadly attacks in the northwestern region of Spain.

Police say on their Twitter feed that they searched the buses in Girona and Garrigas, towns in the northwest of Catalonia, but the operation yielded nothing of importance. They provided no further information.

Authorities launched a manhunt for members of what they believe to be a Catalonia-based extremist group after 14 people were killed and more than 100 injured in attacks in Barcelona on Thursday and in Cambrils, south of Barcelona, on Friday.

Officials say at least one suspect is still at large.

Police say they have identified the five people, presumed to be the attackers, shot to death by police in the Cambrils incident but gave no details.

Four people have been arrested as part of the investigation.