Authorities said the Norwegian frigate, which carried military weapons, ran aground near Bergen after it started taking on water. All crew have been evacuated from the warship.
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.
The Accident Investigation Board said because the tanker is Maltese-registered, the Marine Safety Investigation Unit (MSIU) of Malta will participate in the investigation.
Eight people received minor injuries in the accident, which took place shortly after 4:00 am (0300 GMT) in a busy waterway in the Hjeltefjord near Bergen, Norway's army said.
Eirik Walle, of Norway's rescue centre, told Norwegian news agency NTB the collision smashed a hole in the side of the warship.
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.
It was being towed out of the harbour when it somehow managed to ram the frigate.
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He said: "It is taking in more water than they can pump out".
The frigate is part of a Nato fleet in the Atlantic and the alliance has been informed of the accident, he told reporters.
The sites are operated by Equinor (formerly Statoil), one of the world's largest oil and gas companies.
"The KNM Helge Ingstad suffered damage above and below the waterline".
The KNM Helge Ingstad frigate was launched in November 2007 and measures 134 metres long.
Norway's Accident Investigation Board said a towboat was also involved in the accident.
Several offshore rigs that were pumping their production into the Sture terminal, a major tanker port for crude oil, temporarily suspended the flow, Norwegian newspaper Dagens Naeringsliv reported.