Spanish security officials say they have received more than 600 anonymous tip-offs regarding possible Islamic extremists under a new project set up to better anticipate the threat of attacks.

An Interior Ministry statement said police considered just under half the tip-offs to be of value to security services.

The project launched on December 4 allows people to contact the Intelligence Centre for Terrorism and Organised Crime anonymously through a website, a mobile app or a free telephone hotline. They are monitored 24 hours a day, 365 days a year.

Spain has arrested more than 90 suspected Islamic extremists so far this year.

Europe's worst attack by Islamic militants occurred in Spain in 2004, when bombs on Madrid commuter trains killed 191 people.