A media report said the South has found "decisive evidence" that a North Korean torpedo broke the 1,200-tonne warship the Cheonan in two near the disputed inter-Korean border on March 26, with the loss of 46 lives.
Top officials in the South have dropped widespread hints they believe the North was to blame, although Seoul has not yet formally accused its neighbour.
In a telephone conversation on Monday US time, the White House said, they "emphasised the importance of obtaining a full accounting of the event and committed to follow the facts of the investigation wherever they lead".
The two leaders also called on the North, which abandoned disarmament talks 13 months ago, to "live up to its commitment to eliminate its nuclear weapons programme".
Secretary of State Hillary Clinton will visit Seoul next week in an apparent further show of support.
Lee mounted a multinational probe into the sinking to try to ensure its findings cannot be disputed, while the opposition has accused him of seeking a political advantage before local elections on June 2.
The separate sections of the South Korean warship were salvaged last month from the Yellow Sea -- the scene of previous bloody naval clashes -- and the murky waters were scoured for any fragments of a weapon.
"The analysis of metal pieces and traces of explosive recovered from the Cheonan and the seabed led us to secure decisive evidence that there was a North Korean torpedo attack," Yonhap news agency quoted a military source as saying.
The explosive traces have a similar chemical make-up to substances found in a stray North Korean torpedo secured by the South seven years ago, the source was quoted as saying.
The defence ministry refused comment before the official announcement of the investigation's findings.
Yonhap and other media also said a fragment presumed to be part of the torpedo's propeller had been found. Dong-A Ilbo newspaper said investigators have concluded it was from a torpedo made in either China or Russia.
In a rare move, the North announced Tuesday it has called a meeting of its rubber-stamp parliament meeting for next month -- just two months after the previous session.
Analysts said the meeting may discuss the aftermath of the sinking, which has further fuelled tensions on the divided peninsula.
South Korea has promised a "resolute response" if the North is proved to have been to blame, but is thought to have ruled out a counterattack in favour of trade and diplomatic sanctions.
Seoul officials have said they will likely ask the United Nations Security Council to punish the North. But if was unclear whether the North's ally China, a veto-wielding member, would agree without proof of Pyongyang's involvement.
Defence Minister Kim Tae-Young was to present the final investigation report to security ministers and presidential aides Tuesday afternoon.
Yonhap said the foreign ministry would Wednesday brief the Chinese, Russian and Japanese ambassadors on its contents, a day before it is made public.
South Korea has suspended funding for government-level exchanges with the North. Media reports said it is also considering cutting trade links and may resume anti-Pyongyang loudspeaker broadcasts across the border.
Source: AFP Asian Edition