Up to eight people were injured in the collision in the Hjeltefjord near Bergen, Norway, when the KNM Helge Ingstad crashed into a vessel from Malta.
The warship is currently in danger of sinking, while the tanker, named the Sola TS, is slightly damaged and did not spill oil.
The collision happened this morning at 3am (4am local time) during a training session, officials have confirmed.
Norwegian warship at risk of sinking after collision with oil tanker
Sola TS tanker crashed into Helge Ingstad, tearing a large hole in its side, near the Sture terminal in Oygarden at around 4 a.m (0300GMT) on Thursday.
Andreas Stensoenes, Head of the Royal Norwegian Navy, said: "Helge Ingstad took in water after the collision, lost control and then drifted aground."
The Sola TS did not spill oil, it appears, but the collision forced the shutdown of a North Sea crude export terminal and Norways largest gas processing plant, as well as a number of offshore fields.