KNM Helge Ingstad (F313) is one of the Norwegian navy's five frigates of the Fridtjof Nansen class.
Pictures show the 5,290 ton, 440 foot frigate lying low in the water as it sinks beneath the waves.
Eight people on the KNM Helge Instad were slightly injured, two of whom were taken to a nearby hospital, in the 4am accident in Sture, north of Bergen, said Rear Admiral Nils Andreas Stensoenes, the head of Norway's Navy.
It was being towed out of the harbour when it somehow managed to ram the frigate.
The Norwegian frigate KNM %3CI%3EHelge Ingstad%3C/I%3E takes on water after a collision with the Maltese-flagged tanker %3CI%3ESola TS%3C/I%3E on 8 November 2018 in the Hjeltefjord near Bergen, Norway.
The vessels involved in the accident on Thursday morning were reportedly carrying around 160 people - 137 aboard the navy ship and 23 on the tanker.
Update: Frigate understood to sustain stb bow hull breach with massive water ingress, she's said to develop heavy bow tilt. "The frigate moved to an inshore location and was listing heavily at 11:30am, amidst emergency efforts to prevent the vessel from sinking". Tanker said not to suffer serious damages, as of 0530 UTC she was near collision site, moving in northern direction at dead slow speed, escorted by tug.
The frigate was lying in the water nearly on its side with its stern under the water.
"It took on a lot of water and there is a real danger that it sinks where it is", an official for the rescue centre added.
Some 10,000 liters of helicopter fuel from the frigate has leaked into the sea, said Johan Marius Ly of the Norwegian Coast Guard. Efforts are being made now to prevent the warship, "Helge Ingstad" from sinking.
It is armed with ship-to-ship missiles, torpedoes and machine guns.
The frigate had recently taken part in a North Atlantic Treaty Organisation exercise.
He declined to comment on what would happen to the weapons on board the ship.