However, efforts to salvage the still-submerged bow of the ship hit a snag when one of the chains lifting it from the Yellow Sea snapped because of high waves and strong currents, the office of the Joint Chiefs of Staff said.
South Korean officials say an external explosion caused the March 26 sinking of the 1,200-tonne corvette, which killed 46 sailors in the nation's deadliest peacetime naval tragedy.
"I, as president, will find out the cause of the Cheonan's sinking in full and in detail," Lee said in an emotional speech on public radio.
"I will deal with the results in an unwavering and resolute way, and make sure that such an incident will never recur."
Tearfully reciting the names of all the perished sailors, Lee said: "Your fatherland, which you loved, will never forget you."
South Korea has not directly blamed the communist North for the sinking near their tense maritime border.
Defence Minister Kim Tae-Young has raised the possibility that a mine or torpedo may have hit the Cheonan.
But after weeks of silence, the North's official news agency KCNA said Saturday "the South's puppet military and right-wing conservative politicians are now making a foolish attempt to link the tragedy to us".
"The reason that the South's puppets are claiming the North's involvement is also linked to their foolish efforts to put pressure on us, even by stirring up international opinion in favor of sanctions," KCNA said.
Yoon Duk-Yong, co-head of the South's investigation team, said Friday that the warship appeared to have received a powerful impact on the port side, where steel plate was curved inwards.
He ruled out an onboard blast or a shipwreck.
Navy officials said the investigators were at a naval base south of Seoul, looking into the Cheonan's recovered stern and debris for more clues.
"Up to now, no other debris -- except the warship wreckage -- have been found," Seoul's defence ministry spokesman Won Tae-Jae said Monday.
However, salvage operations were hampered when one of three chains around the bow of the ship snapped on Sunday, officials said.
"It appears the chain could not withstand the tension from high waves," naval officer Park Sung-woo said.
An expanded 12-strong US team of experts led by Rear Admiral Thomas J. Eccles were joining the investigation at the naval base in Pyeongtake, 70 kilometres (40 miles) south of Seoul, the defence ministry said.
It said the US team included three experts who had investigated the 2000 bombing of the USS Cole destroyer in Yemen.
Analysts have said the incident could harm diplomatic efforts aimed at persuading North Korea back to six-party nuclear disarmament talks that the communist state quit a year ago.
The Yellow Sea border was the scene of deadly naval clashes between the North and South in 1999 and 2002 and of a firefight last November that left a North Korean patrol boat in flames.
Source: AFP Asian Edition