South Korea on Thursday was still struggling to explain last week's naval disaster in the Yellow Sea, but played down speculation a North Korean submarine was involved.
Anguished families of 46 missing sailors have pressed for answers since the 1,200-tonne corvette broke in two near the tense border and sank Friday night.
Defence ministry and presidential officials dismissed media reports that the Cheonan had been tracking North Korean submarines at the time.
The ministry said in a statement it was not excluding any possible cause.
But it said it had not detected any North Korean submarine activity near the area at the time and believed "the possibility of their infiltration is very low".
Seoul has ordered its military on alert but not cited any evidence the North was involved. The defence minister has said a North Korean mine -- either drifting or deliberately placed -- might have caused the disaster.
The disputed border was the scene of deadly naval clashes in 1999 and 2002 and of a firefight last November.
A spokesman for the Joint Chiefs of Staff told AFP mine-detecting vessels were searching the area for any fragments of the ship or of a mine or torpedo.
He said a 60-member investigation team of military and civilian experts, including specialists in naval weaponry, explosives and shipbuilding engineering, was being formed.
President Lee Myung-Bak, who has ordered the military and civil service on heightened alert, said the country's credibility was at stake.
"There should be no suspicion or negligence" in the investigation or the government's affairs alike, he said. "That is the way for South Korea to gain trust in the international community."
US President Barack Obama called Lee "to offer his support and condolences", the White House said.
US ships are already taking part in a major salvage and rescue operation off Baengnyeong Island, which was suspended Wednesday and Thursday because of rough seas.
Divers battling strong currents and frigid murky waters have not yet been able fully to search the separate hull sections.
Media reports said they had found the hull was split cleanly, as if with a knife, although there were conflicting theories on what this might mean.
The Kyunghyang daily quoted one expert as saying the clean edges suggested metal fatigue in the 21-year-old ship. Chosun Ilbo newspaper said the shock wave from a torpedo could have caused welds to open.
The defence ministry said the ship was taken out of the water in 2008 for regular maintenance and no signs of wear were detected.
Authorities have reported the sound of an explosion before the sinking. One expert, however, told AFP he believes the warship may have grounded.
"My scenario is this: the ship grounded in the shallow water which is not usually navigated and took in water from the damaged bottom," said Paik Jeom-Ki, a Pusan University professor of ship mechanics.
"When the stern lost flotation and became weighed down with water, the ship broke into two. The sound of metal being torn apart may sound like an explosion."
Yonhap news agency said the ship's stern was damaged, although not severely, during the 1999 naval battle with the North.
A total of 58 people were rescued from the bow section of the 88-metre (290-foot) ship soon after the sinking.
Hopes of finding more survivors faded Monday when divers heard no response after banging on the two sections of the sunken hull.