The men were detained following co-ordinated raids on locations including an internet cafe [AFP]

Dutch police have released 11 of 12 Somali men arrested in Rotterdam for alleged "terrorism" offences ahead of Christmas, while one man remains in custody, according to Dutch prosecutors.

Some of the freed men have been handed over to immigration authorities, prosecutors said in a statement posted on their website on Tuesday.

Two are still suspects, prosecutors said.

The men, all aged between 19 to 48, were detained after co-ordinated raids on locations including a hotel and an internet cafe on December 24.

The raids followed a warning by AIVD, the Dutch intelligence service, that several Somalis were planning an attack in the Netherlands.

No weapons or explosives were found, prosecutors said, and the intended target was unknown.

Dutch newspaper De Telegraaf, quoting an unidentified intelligence source, said some of the Somalis arrested had planned to shoot down an Apache helicopter at the nearby Gilze-Rijen air base using a rocket-propelled grenade.

There has been speculation that, as Europe's biggest port and a large oil storage base, Rotterdam is strategically important and a potential target for attack.

Dutch intelligence declined to comment.

'Possible attack'

The prosecutors' statement said that the arrests on Friday "focused primarily on preventing a possible attack".

"The arrests followed on an AIVD message, which reported that a number of Somalis wanted to commit a terrorist attack in the Netherlands on December 24 or 25," it said.

Western authorities have been on high alert since a Middle East-born man blew himself up in Stockholm two weeks ago and a senior Iraqi official issued a warning that al-Qaeda was planning attacks in the United States, Britain and Europe.

Despite the arrests, the Dutch National Terrorism Co-ordinator left the terrorist alert level unchanged, indicating the likelihood of an attack was "limited".

But Dutch intelligence services have reportedly been closely watching the growing Somali community in the Netherlands.

One US citizen of Somali extraction is under arrest and is fighting extradition to the US, suspected of supplying money to the al-Shabab group for weapons and to finance trips for potential recruits.

The US state department and many European countries consider al-Shabab a terror group with links to al-Qaeda.

Source: Agencies