Norway's intelligence service PST said Friday it had arrested a 16-year-old Syrian immigrant suspected of planning an attack on an undisclosed target.

The suspect, who was arrested on Thursday, has been living in Oslo for several years and had begun preparations to carry out his attack, PST spokesman Trond Hugubakken told AFP.

"He is suspected of plotting an attack. It's more serious than the usual cases of participation in or attempted participation in a terrorist organisation," he said.

The teen was to appear before a judge on Friday for a detention hearing, which PST was to request be held behind closed doors.

No details were provided on the intended target which, according to information provided to AFP, was in Norway.

"He claims he is innocent and will ask to be released. His age is not compatible with a detention order," the suspect's lawyer Andreas Berg Fevang told AFP.

According to the VG newspaper, the teen is an Islamic State supporter who came to Norway as part of a family reunification.

"In recent years we have seen youngsters, including kids as young as 13 or 14, being radicalised in both far-right circles and radical Islamist circles," Hugubakken said.

"It's a worrying trend."

The PST did not exclude further arrests.

Also on Friday, Norwegian police charged a 30-year-old Norwegian woman of Pakistani origin, repatriated last year from Syria's Al-Hol camp for humanitarian reasons, with participating in a terrorist organisation.

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