German law enforcement officials say 11 people were arrested in western Germany on suspicion of planning an Islamic extremist terror attack.

The dpa news agency reported the suspects were taken into custody on Friday and Saturday in the towns of Essen, Duesseldorf, Wuppertal, Moenchengladbach, Duisburg and Ulm, citing a spokesman for prosecutors in Duesseldorf.

The prosecutors' spokesman was quoted as saying Saturday that the group was suspected of having ties to Islamic State militants, either as sympathisers or a splinter group, and of planning a 'serious act of violent subversion', although there was no indication of a specific target.

The prosecutor's office said the suspects were ordered released but the investigation would continue.

Officials said one suspect was from Tajikistan but provided no information about the nationalities of the others.