South Korea's defence minister said Friday that a torpedo might have sunk a warship near the tense North Korean border but cautioned that the investigation was incomplete.

"A torpedo or a sea mine might have been involved but a torpedo is a more realistic cause than a mine," minister Kim Tae-Young told parliament as divers resumed a search for 46 sailors missing from the 1,200-tonne corvette.

Sailors operating the ship's sonar detection system had not detected any approaching torpedo, Kim said, adding: "Patience is needed until investigation results come out."

A week after the disaster, officials are still groping for answers as to what caused the ship to break in two in the Yellow Sea on the night of March 26.

Earlier in the week Kim had said a North Korean mine -- either drifting or deliberately placed -- may have caused the blast. He did not say Friday why he now thought a torpedo more likely and did not indicate who might have fired one.

The disputed border was the scene of deadly naval clashes between North and South Korea in 1999 and 2002 and of a firefight last November.

The minister told legislators the chances of an explosion inside the warship were slim, although he did not rule them out. Addressing another theory, he said metal fatigue was unlikely in the 21-year-old ship.

Seoul has ordered its military on alert but not cited any evidence the North was involved.

Fifty-eight crewmen were saved soon after the sinking, but no one since then. Officials suspect most of the missing are in the rear section of the hull.

After suspending work for two days because of high waves and strong currents, US and South Korean divers Friday resumed attempts to explore the hull sections.

"It is very hard to say when we will be able to find the missing," military spokesman Lee Ki-Shik told a briefing earlier in the day. "Our rescuers are still trying to get in despite the bad weather, so please be patient."

A fishing boat with nine people on board helping the search went missing Friday night and was feared to have sunk, Yonhap news agency reported.

Police said they lost contact with the vessel after receiving a distress signal from the area where the warship sank, it said.

No one has officially declared the missing sailors to be dead, even though the air in any watertight compartments would likely have been used up.

Rescuers have pumped oxygen into the hull but Kim acknowledged Friday the operation "is not working very well".

Apart from establishing the fate of the missing, authorities are desperate for clues to the cause of the disaster.

Officials have said these may not emerge until the hull sections are lifted from the murky seabed. A private salvage ship is at the scene.

President Lee Myung-Bak appealed Friday for people not to overreact.

"North Korea and the international community are watching. We should look into the case in a calm manner and use it as a chance to elevate national capability," he said.

earlier related report
Obama offers support, condolences to SKorea over sunken ship
Washington (AFP) March 31, 2010 - US President Barack Obama called South Korean President Lee Myung-bak on Wednesday to offer "support and condolences" following the sinking of a South Korean navy ship, the White House said.

South Korea on Wednesday ordered all government officials to stay on emergency alert until the crisis sparked by the mysterious sinking of a 1,200-tonne navy corvette late Friday near the tense border with North Korea is resolved.

Obama called Lee "to offer his support and condolences following the sinking of the South Korean navy vessel Cheonan on March 26," the White House statement read.

The US president also told Lee "that the thoughts and prayers of the American people are with the families of those missing and with the family of the South Korean navy diver who died trying to rescue them."

Fifty-eight people were rescued from the bow section of the 88-meter (290-foot) ship soon after the sinking, but 46 sailors remain missing.

Dozens of divers have braved the Yellow Sea's strong currents and frigid and murky waters, trying in vain to get inside the hull of the sunken ship. One of the divers, Han Joo-Ho, died Tuesday.

Obama also noted "that the US Navy had extended assistance to South Korea's ongoing search and recovery effort and said we were prepared to provide further help if needed."

Seoul has not cited any evidence the North was involved, although the defense 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 a fire fight last November.

Obama and Lee also discussed the upcoming Nuclear Security Summit in Washington on April 12-13, the White House said.