Spanish police have arrested eight people and raided 12 properties in Barcelona in an operation against suspected Islamist militants carried out in conjunction with Belgian police, Reuters has reported.

Quoting a Spanish police spokesman, the agency said that some of those arrested were suspected of links to the Belgian suicide bombings in March of last year, which killed 31 people at Brussels airport and on a metro train.

"The detained were all Moroccans aged between 21 and 39," he said.

The raids follow an eight-month investigation of a group of suspected Islamist militants with links to organised crime in the Barcelona area, the police said in a statement. The investigation remains open.

Not including Tuesday's arrests, Spain has detained 22 people in 16 separate operations this year in connection with Islamist extremism, Reuters added.