A Norwegian navy frigate and a Maltese oil tanker collided in a fjord in western Norway on Thursday, local authorities said, with seven people receiving minor injuries.
The KNM Helge Ingstad frigate, which was returning from NATO's Trident Juncture exercises, was evacuated after the collision with the Sola TS tanker, Norway's army said.
"It took on a lot of water and there is a real danger that it sinks where it is," an official for the Sola rescue centre told AFP.
An image published by Norwegian broadcaster NRK showed that water had almost reached the level of the frigate's helicopter platform.
A total of 137 people were on board the frigate while 23 were on the tanker, which was flying the Maltese flag, the official said.
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.
"A small oil slick from the frigate has been detected but nothing large," the official added.
The circumstances of the accident, which took place shortly after 4:00 am (0300 GMT) in the Hjeltefjord near Bergen, are not yet clear.
"The armed forces is now reviewing all the means available in the region to assist the KNM Helge Ingstad," Lieutenant Colonel Ivar Moen told AFP.
Built in 2009, the KNM Helge Ingstad participated in chemical disarmament operations in Syria between December 2013 and May 2014.