BARCELONA - A manhunt was underway for the driver of a van that mowed through crowds of tourists on Barcelona's most famous avenue on Thursday, killing at least 13 people in an attack that was claimed by Islamic State.

Two other terror-related incidents were reported in Spain on Thursday.

Authorities said the death toll from the van carnage could rise, with more than 100 people injured.

Police said they arrested two men, a Moroccan and a man from Spain's north African enclave Melilla, though neither was the driver, whom witnesses said fled on foot.

Another terror attack was reported in Cambrils, 73 miles south of Barcelona along the Mediterranean coast.

And hours before the van driver's rampage, one person was killed in an explosion in a house in a town southwest of Barcelona, in an incident linked to the attack, police added. Residents of the house were preparing explosives, a police source said.

Police said four attackers were killed and one injured in a shootout during an operation against what they called a possible "terror attack" in Cambrils.

The Guardian of London reported that five people were injured on the seafront Paseo Maritimo in Cambrils.

Witnesses said the white van zigzagged at high speed down Las Ramblas, a busy avenue thronged with tourists, ramming pedestrians and cyclists, sending some hurtling through the air and leaving bodies in its wake.

Islamic State's Amaq news agency said: "The perpetrators of the Barcelona attack are soldiers of the Islamic State and carried out the operation in response to calls for targeting coalition states".

If the involvement of Islamist militants is confirmed, it would be the latest in a string of attacks in the past 13 months in which they used vehicles to bring carnage to European cities.

That modus operandi has killed well over 100 people in Nice, France; Berlin, London and Stockholm.

British tourist Keith Welling, who arrived in Barcelona on Wednesday with his wife and 9-year-old daughter, said they saw the van drive past them down the avenue and took refuge in a restaurant.

"People were shouting, and we heard a bang and someone cried that it was a gunshot. ... Me and my family ran into the restaurant along with around 40 other people."

"At first people were going crazy in there, lots of people crying, including a little girl around 3 years old."

It was the deadliest attack in Spain since March 2004, when Islamist militants placed bombs on commuter trains in Madrid, killing 191 people and wounding more than 1,800.

Spanish Prime Minister Mariano Rajoy announced three days of official mourning for what he called a "jihadist attack".

US President Donald Trump said: "The United States condemns the terror attack in Barcelona, Spain, and will do whatever is necessary to help." He added: "Be tough & strong, we love you!"

Police said the two men detained on Thursday had been arrested in two towns, Ripoll and Alcanar, both in the region of Catalonia, of which Barcelona is the capital.

A spokesman for the first minister of Barcelona, Artur Mas, tweeted an image of a Catalan flag with the slogan: "We are stronger and we will win because our values are stronger and are winners. Stay strong Barcelona, visca Catalunya!"

Bo Leung in London contributed to this story.

Reuters