The KNM Helge Ingstad frigate was almost totally submerged in a fjord near the western city of Bergen after it collided with an oil tanker in November.

Investigators said the frigate's crew appeared to have confused the Maltese tanker with land.

Authorities deployed four cranes and two barges to salvage the ship, with favourable weather conditions allowing the operation to go ahead.

The operation is expected to take several days, the Norwegian army said.

Eight of the 137 people aboard the frigate suffered minor injuries when the it crashed returning from NATO exercises.

Authorities plan to load the ship on to a semi-submerged barge, unload its weapons and then transport it to a military port to determine whether it can be repaired.

The operation is expected to cost at least 500 million krone (51 million euros, $58 million), according to Norwegian press reports.