A rock concert in Rotterdam was cancelled on Wednesday evening due to a terror threat, Reuters reports.

A bus with Spanish license plates and containing gas bottles was found near the concert hall where the concert was cancelled, the city's mayor, Ahmed Aboutaleb, later said.

Aboutaleb told a news conference it wasn't clear whether the threat and the bus were connected. The bus driver was taken into police custody for questioning, he added, according to Reuters.

Police told the Dutch news agency ANP that the concert, which was to feature Californian band called "the Allah-lahs", was canceled after a tip from Spanish police.

Spain has been under a heightened terror alert this past week following two car ramming attacks in Barcelona and the resort town of Cambrils.

On Tuesday, a suspected member of the terror cell that carried out the attacks in Spain admitted to a judge the jihadists had planned to hit monuments in an even bigger attack.

Mohamed Houli Chemlal, 21, told the judge that the cell was planning "an attack on an even greater scale, targeting monuments" using bombs.