The car used in the Cambrils attack overturned. Photo: AFP

Catalonia's police chief said blasts in a town south of Barcelona on Wednesday deprived the plotters of bomb material, forcing them to carry out simpler attacks using vehicles to ram crowds.

Police are seeking Moussa Oukabir, thought to be 17, suspected of carrying out Thursday's Barcelona attack.

Thirteen people died in Las Ramblas.

Scores of others were injured as a van driver zig-zagged along the famous tourist street.

Moussa Oukabir is suspected of using his brother's documents to rent the van that mowed down people on the famous boulevard.

Hours later, police killed five suspected jihadists in a second vehicle attack in the town of Cambrils. A woman injured in that attack died later.

Police said the men killed in Cambrils were linked to the Barcelona attack, which the Islamic State (IS) group said it had carried out.

Spanish Prime Minister Mariano Rajoy condemned what he called a "jihadist attack" in Barcelona. He has announced three days of national mourning and a minute's silence was held at noon (10:00 GMT) on Friday.

What lines of inquiry are police following?

Police say they are investigating the theory that the attacks were being prepared over a substantial period of time in a private house in Alcanar, a small town south of Barcelona.

Explosions ripped through the building on Wednesday evening, apparently caused by exploding gas canisters. One person was killed.

The explosives were being prepared for use in one or more attacks in Barcelona, police said.

Catalan Police Chief Josep Lluis Trapero said the actual attacks in Barcelona and Cambrils were "rudimentary" in comparison.

Who is being sought?

The driver of the van fled on foot after mowing down tourists and locals on Las Ramblas.

Spanish police say that they are seeking Moussa Oukabir, two images of whom have been published.

Catalonia's Interior Minister Joaquim Forn, quoted by the Associated Press, said: "We had local police on the scene, but we were unable to shoot him, as the Ramblas were packed with people."

Later Police Chief Trapero said it was suspected that the driver of the Barcelona van was one of the five killed in Cambrils, though it was not clear that Moussa Oukabir was behind the wheel.

Spanish media say Moussa Oukabir rented two vans, the one used in the attack and another found hours later in the town of Vic, north of Barcelona, that was intended as a getaway vehicle.

His brother, Driss Oukabir, was arrested after reportedly turning himself in on Thursday.

Reports suggest Driss, who is in his 20s and was born in Morocco, told police he was not involved and that his documents had been stolen.

Three other arrests have been made. Two of the men are Moroccan, and the third was born in Melilla, the autonomous Spanish city on the north coast of Africa.

Police said three of the arrests were made in the Catalan town of Ripoll and one in Alcanar after Wednesday's explosions.

They gave the ages of the arrested men as 21, 27, 28 and 34.