South Korean officials say an external blast sank the navy ship

An "external explosion" probably sank the South Korean naval vessel which went down near North Korean waters last month, an investigator says.

"The possibility of an external explosion is far higher than that of an internal explosion," Yoon Duk-yong told a news conference in Seoul.

He was speaking a day after the stern section of the ship was raised from the seabed.

Fifty-eight crew survived, but more than 40 sailors died.

Salvage workers found 36 bodies in the shattered hull of the Cheonan, a 1,200-tonne navy gunboat, which sank in mysterious circumstances three weeks ago.

Two more bodies were recovered earlier, and another eight sailors remain unaccounted for.

The bow section of the vessel is due to be raised in about 10 days' time.

'Fair probe'

The Cheonan sank close to the sea border which marks North and South Korean territorial waters.

South Korean officials have previously suggested two possibilities for the blast.

They say the ship could have struck an old mine left over from the 1950-1953 Korean War, or it could have been sunk by a torpedo fired by North Korea.

"The remaining task is to find the cause of the incident through a scientific and fair probe," said Prime Minister Chung Un-chan.

North Korea has made no official comment on the incident, which comes at a time of tensions between the two nations.

It does not accept the maritime border, known as the Northern Limit Line, which was drawn unilaterally by the US-led United Nations Command at the end of the Korean War.

The sea border has been the scene of deadly clashes between the navies of the two Koreas in the past.