Adm. Nils Andreas Stensoenes, the head of the Royal Norwegian Navy, said Thursday during a news conference. The frigate's 137 crew members were evacuated amid fears that it may sink.
The tanker, the Sola TS, was only slightly damaged and it appears that it did not spill oil.
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.
The Maltese-flagged vessel had left Equinor's Sture oil shipment terminal with a cargo of North Sea crude, Reuters news agency reports. Both Norway's Accident Investigation Board and Malta's Marine Safety Investigation Unit will investigate the collision, as the tanker was registered in Malta.
Update: Frigate understood to sustain stb bow hull breach with massive water ingress, she's said to develop heavy bow tilt.
Pictures show water has also overwhelmed the frigate's helicopter platform on its stern.
The 62,000-tonne tanker received only slight damage and is waiting to be towed to a nearby oil terminal, the rescue centre official said.
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The KNM Helge Ingstad frigate was launched in November 2007 and measures 134 metres long.
The two vessels collided at about 04:00 local time (03:00 GMT) as the frigate was sailing inner fjords for training, officials say.
"The armed forces is now reviewing all the means available in the region to assist the KNM Helge Ingstad", Lieutenant Colonel Ivar Moen said.
Update: 0700 UTC - according to latest reports frigate was taken to shallow waters to avoid sinking, water ingress is incontrollable.
The Norwegian frigate KNM Helge Instad is in danger of sinking after being rammed by an oil tanker while returning from the NATO Trident Juncture exercise, according to The Associated Press.
An image of the ship tipped to one side was posted with the comment.