OSLO, Feb. 26 (Xinhua) -- Norway started on Tuesday the operation to raise the sunken KNM Helge Ingstad frigate that had collided with an oil tanker off Norway's western coast more than three months ago, the military said.

The operation was estimated to take five to six days, according to a statement published by the Norwegian Armed Forces.

"The operation to raise the ship out of the water has been complex, and it has been postponed several times due to rough weather conditions," the statement said.

Two barges have anchored up next to the frigate and four large hoists on the two barges will now slowly lift the frigate from the seabed.

After it is raised, the frigate will be shipped to Haakonsvern Naval Base, Bergen.

The Norwegian navy's frigate KNM Helge Ingstad collided with the Malta-registered oil tanker Sola TS on Nov. 8. near Bergen, Norway.

The frigate took a lot of water as the collision led to a large hole in its side. Eight people on board the navy ship suffered minor injuries while the tanker sustained only slight damage.

The Norwegian navy frigate sank almost completely under water several days after the collision.

In late November, investigators ruled out technical errors in the collision, saying the "preliminary assessment is that the accident was not caused by any single act or event, but can be explained by a series of interacting complex factors and circumstances."