Dutch justice authorities were Monday mulling their next move after the weekend arrest of 12 Somali men on terrorism suspicions, amid criticism of the country's secret service after five of the men were quickly released.

Reports said the remaining seven suspects might appear before an arraigning judge later Monday or on Tuesday.

The 12 Somali men aged between 19 and 48 years were arrested on Christmas Eve in the port city of Rotterdam after the secret service AIVD said it had concrete evidence of planned attacks.

After a police swoop of an internet cafe, four flats and two hotel rooms, agents seized 11 computers, but found no explosives or weapons, and the target of the planned attacks remained unclear, prosecutors conceded.

Relatives and friends of the 12 detainees had protested their innocence in remarks to the media.

On Sunday, five of the suspects were released after police questioning turned up no evidence to back the allegations.

Lawyers Monday criticised the AIVD and demanded an independent investigation.

'There can be no other explanation except for a mistake on the part of the AIVD that the suspects had to be released again so quickly,' lawyer Michael Ruperti told reporters.

Meanwhile de Volkskrant newspaper cited sources in the Netherlands, Kenya and Somalia as saying that several of those detained were linked to the militant Islamic group Al-Shabaab in Somalia.

One of the 12 arrested was related to Al-Shabaab commander Mohammed Garmashago, who is believed to be connected to the al-Qaeda network, the newspaper reported.