The formal accusation is expected to be announced on Thursday and South Korea will ask the UN Security Council to take up the matter, The Washington Post said citing sources.

Barack Obama, the US president, and Lee Myung-bak, his South Korean counterpart, had discussed the ship sinking incident by telephone on Monday night, but did not disclose details of the investigation.

Gunpowder evidence

The report comes amid a growing chill in relations between the rival Koreas, which remain technically at war under a truce that ended fighting in the 1950-1953 Korean War.

The 1,200-tonne ship was sunk by an external explosion near a disputed sea border [AFP]

North Korea denies involvement, but suspicion has focused on Pyongyang given its history of attacks on the South.

South Korea's Yonhap news agency said on Tuesday citing an unidentified military official that investigators have obtained traces of explosives from the warship's wreckage that are identical to gunpowder ingredients of a North Korean torpedo.

The South Korean defence ministry said it could not confirm the report, but said it had collected a stray North Korean torpedo that came ashore off the west coast seven years ago.

Earlier on Tuesday, South Korea's main newspapers, including the Chosun Ilbo,
also reported that investigators found fragments of a torpedo propeller near the sinking site that appeared to have been part of a Soviet- or Chinese-made

Meanwhile, Hillary Clinton, the US secretary of state, has met special US envoys to North Korea, Stephen Bosworth and Sung Kim, on Tuesday.

Clinton will be in Seoul next week at the end of her Asia tour for talks likely to be dominated by North Korea's nuclear programme and the Cheonan report with Japanese, Chinese and South Korean leaders.