The operation has begun to raise a Norwegian naval frigate that sank last year in a harbor north of Bergen following a collision with an oil tanker.

Anders Penna of the salvage company in charge of raising the KNM Helge Ingstad says it is "very complex and demanding operation," adding it could take up to six days.

Penna says the plan is to put the frigate on a barge and transport it to a navy base where the damage will be assessed.

Two giant cranes on Tuesday began raising the 134-meter (442-foot) vessel that collided Nov. 8 with Maltese-flagged oil tanker Sola TS, tearing a large hole in the frigate's side.

The cause of the accident has not yet been established. No one was injured.