One Norwegian news outlet reports that according to tracking data, the frigate's geolocation system was turned off at the time of the collision, noting, however, that the armed forces of Norway have neither confirmed nor denied that.
The 137 people on board the KNM Helge Ingstad frigate, which was returning from NATO's Trident Juncture exercises, have been evacuated after the collision with the Sola TS tanker, the army said.
The KNM Helge Ingstad frigate took on water and was at risk of sinking.
According to reports from the rescue crews, seven people have been lightly injured in the collision, and there has been no oil spilled from the tanker, which suffered a lot less damage than the frigate, which was returning from a North Atlantic Treaty Organisation military drill.
Norwegian news agency NTB is reporting that the collision tore a hole in the frigate's side, and that it is taking in more water than can be pumped out.
In the early afternoon, the grey 5,000-tonne vessel was listing heavily on its side, its helicopter landing pad at the back of the ship lying largely under the water, television images showed.
The Maltese oil tanker was carrying about 625,000 barrels of crude, but officials said none spilled after Thursday's collision.More news: Russian jet intercepts US Navy aircraft over Black Sea
Norway's Accident Investigation Board are trying to establish what exactly happened to trigger the crash.
The Sture oil terminal and the Kollsnes gas processing plant were shut down as a precautionary measure.
The Navy quickly evacuated all the crew except for 10 essential staff members to prevent a possible fuel leak.
Comment: Again, laden massive cargo ship collided with Navy frigate, similar to accidents which occurred with USA Nay frigates in Japan and off Singapore, recently.
The tanker, with 23 crew, suffered damage well above the waterline.
The 134-meter (442-foot) long frigate, built in Spain in 2009, has a helipad platform on its stern.
The £500 million Helge Instad was launched in 2007 and is one of five Fridtjof Nansen-class frigates operated by Norway.