The 3.5 meter (11.5 foot) long, two-ton sub, a "Battlespace Preparation Autonomous Underwater Vehicle" (BPAUV), disappeared last Thursday while charting the sea bed and looking for mines during a test run of the new vessel under tough conditions.

The BPAUV was unveiled by the US Navy last June. Norwegian defense officials had waited for US clearance to announce that the vessel had gone missing.

The USS Swift, a mine seeking vessel, had to break off the exercise after losing contact with the sub.

Specially trained dolphins, equipped with cameras, are being used to try and find the sub. The dolphins have carried out searches for mines and, primarily hostile, mini-subs before.

"The sub follows a pre-programmed pattern and so is not remote controlled. The idea is that it is supposed to come up again at a given place. But it didn't," Commander Thom Knustad of central defense headquarters said.

Knustad said that if the sub was lying on the sea bed it would be at least as hard as finding a needle in a haystack.