The Northern Territory Attorney-General has confirmed the NT Police and the Australian Federal Police (AFP) are running a joint investigation into a Darwin teenager to see if he has broken any terrorism laws.

The AFP also confirmed it was making enquiries relating to a 16-year-old male in Darwin.

A spokesperson said he had not been charged with any Commonwealth offence and no further comment would be made.

Attorney-General John Elferink said the teen was first arrested by NT Police for aggravated robbery and stealing offences.

"During the investigation of that, terrorism-related material, namely a list of targets and evidence that he had been accessing ISIS websites on his mobile device raised the issue of what he was actually doing," he said.

"It's not known whether he received any information back from ISIS. We are presuming as a Government the worst-case scenario ... that's part of the investigation, whether this young man has partnered up with anyone else.

"It might the case that he's just being a very silly young man making some very poor decisions, or he may be more committed to the cause.

"That's the tragedy of the ISIS model: a disaffected young person can become a jihadist.

"The difference often between a jihadist and a disaffected child is the mood that the child is in at the time when they're holding a knife in their hand."

The Attorney-General confirmed the boy is being held in the adult prison at Holtze.

"We as the Northern Territory Government are not taking this matter lightly," he said.

NT Police have declined to give any further information, given the age of the youth and the fact the matter of the stealing charges is before the court.

They referred all aspects of "other particulars" to the AFP.