BERLIN (REUTERS) - German police investigating four people suspected of planning an Islamist-motivated attack raided nine locations in Berlin and the eastern state of Saxony Anhalt on Thursday (Dec 14), prosecutors said.

The four suspects, aged between 18 and 21, are accused of being members of the Islamic State in Iraq and Syria (ISIS) group, prosecutors and police said in a statement.

Three are believed to be in Syria: two travelled from Berlin via Istanbul to ISIS-held territory in Syria in November 2016 and a third is accused of receiving military training in Syria. The fourth is believed to have helped the other three travel there.

Prosecutors did not say if any arrests were made in the raids, mounted by some 130 officers, including special forces, who confiscated electronic devices.

"The General Prosecutor's Office in Berlin is investigating four suspects aged between 18 and 21 who are suspected of membership in a terrorist organisation (ISIS) as well as of preparing a serious crime against the state," the statement said.

Bild newspaper said police had arrested a number of people suspected of having links to a failed asylum seeker who killed 12 people by driving a truck into crowds at a Berlin Christmas market last year.

Tunisian Anis Amri escaped after launching the Dec 19 attack and was shot by Italian police in Milan less than a week later. The affair exposed failings by intelligence agencies who had stopped surveillance of Amri after concluding he posed no danger.

Security at Christmas markets has been beefed up this year with guards and concrete blocks.