Police in the Dutch port city of Rotterdam detained the driver of a van with Spanish licence plates and a number of gas tanks inside after a concert by a California rock band was cancelled Wednesday night following a threat, the city's mayor said.

A Spanish counterterrorism official said the van is not connected to the attacks that killed 15 people in Spain last week.

The official said investigators discarded a possible link between the van's driver and the extremist cell that carried out the attacks in and around Barcelona after questioning the driver and examining the van.

Rotterdam police could not be reached for a reaction to the Spanish official's comments.

Police detained the driver for questioning about two hours after a concert by Los Angeles band Allah-Las was called off after Dutch police received a terror warning. Rotterdam Mayor Ahmed Aboutaleb said the warning came from Spanish police.

However, at a hastily arranged news conference, Aboutaleb said that it was too soon to link the van to the unspecified threat directed at the concert.

"It is not yet good to say that the things are linked," Aboutaleb said. "It still has to be established that there was a link between the van and the threat. We can't say that yet."

Netherlands Concert Cancelled

Rotterdam Mayor Ahmed Aboutaleb speaks to the media in Rotterdam, Wednesday, after the concert. (RTL/Associated Press)

Military explosives experts checked the van and a police officer later got behind the wheel and drove the vehicle away, Dutch broadcaster NOS reported.

Police said in a statement that the van contained "a couple of gas bottles."

The Dutch terror threat level, which is at level four on a five-step scale, remained unchanged, Lodewijk Hekking, a spokesperson for the country's counterterror chief told The Associated Press in a text message.

The Spanish counterterrorism investigation official said Spain's Civil Guard received "an alert indicating the possibility of an attack today in a concert that was going to take place in Rotterdam."

The Civil Guard shared the information with Dutch authorities and is investigating the threat, said the source, speaking anonymously because the Civil Guard is still probing the threat.

Police in Spain have been investigating the deadly vehicle attacks last week in and around Barcelona. They later found bomb-making equipment that included more than 100 tanks of butane gas, nails and 500 litres of acetone.

Rotterdam police said they took the information about a threat "seriously enough that after discussion with organizers it was decided to cancel the event."

Concert organizer Rotown said earlier on Twitter that the concert venue, a former grain silo called Maassilo, was being evacuated because of the unspecified threat. The show hadn't yet started when the decision to cancel it was made.

Netherlands Concert Cancelled

Miles Michaud, lead singer of the California band Allah-Las, performs in 2016. A rock venue in the Dutch port city of Rotterdam says it has cancelled a concert by the rock group due to a threat. (Dario Lopez-Mills/Associated Press)

Dutch television showed officers in body armour outside Maassilo and what appeared to be members of the band leaving the venue in a white van with a police escort. By around midnight, the area was calm and police had lifted the cordon they earlier put up around the venue, the NOS reported.

Allah-Las is a four-piece band from Los Angeles.

In an email to The Associated Press, the band's label, Mexican Summer, said: "Due to a potential terror threat at The Maassilo in Rotterdam, the Allah-Las show was cancelled tonight.

It added: "Details are not available at this time as the incident is still under investigation. The band is unharmed and are very grateful to the Rotterdam Police and other responsible agencies for detecting the potential threat before anyone was hurt."

In an interview with British newspaper The Guardian last year, band members said they chose the word Allah — Arabic for God — because they were seeking a "holy-sounding" name and did not realize it might cause offence.

"We get emails from Muslims, here in the U.S. and around the world, saying they're offended, but that absolutely wasn't our intention," lead singer Miles Michaud told the newspaper. "We email back and explain why we chose the name, and mainly they understand."