South Korea said Wednesday it has ordered all government officials to stay on emergency alert until the crisis sparked by the mysterious sinking of a warship is resolved.

A media report said the 1,200-tonne corvette, which was torn in two by a blast Friday night near the tense border with North Korea, had been on a mission to track the North's miniature submarines.

South Korean officials have been told not to take leave and to stay alert even when off-duty in case of emergencies, the home ministry said, reiterating an instruction first issued Saturday.

The 655,000-strong military and the police force were also ordered on heightened readiness, after the sinking in the Yellow Sea which left 46 sailors missing.

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

Navy chief Kim Sung-Chan has said the ship's munitions storage room did not appear to have exploded and "the ship was broken in two because of powerful outside pressure or an (exterior) explosion".

YTN television, quoting a military source, said the corvette had been tracking North Korean miniature submarines on the day it sank.

It said three or four submersible craft had been seen moving southwards and South Korea deployed its ship in response.

But the television said the circumstances did not necessarily indicate the sinking was directly related to the submersibles.

The defence ministry declined comment on the report.

Dong-A Ilbo newspaper said US and South Korean intelligence had satellite photos showing submersible craft moving in and out of a west coast base at Sagot in North Korea before and after the sinking.

"North Korean submersible or semi-submersible craft often disappear and return, and it is difficult to link it to the incident in a decisive manner," it quoted a Seoul government source as saying.

A huge search for the missing sailors, which has claimed the life of one naval rescue diver, was suspended Wednesday due to stormy seas.

The military officially refuses to abandon hope but officers said privately there was no chance anyone could still be alive in watertight compartments inside the sunken hull.

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 off Baengnyeong island. But angry and tearful relatives have been demanding swifter rescue action.

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

Defence ministry spokesman Won Tae-Jae said waves Wednesday were up to 2.5 metres high and winds and currents were strong. He said divers had managed to open some hatches but had not penetrated inside the hull.

Grieving relatives accused the military of acting too slowly and said efforts must continue until all the missing are found.

"During our visit to the site, we saw that efforts to search the ship's stern and rescue survivors were delayed because a rescue ship was not promptly dispatched," Yonhap news agency quoted a family representative as saying.

earlier related report
N.Korea's Kim likely to visit China soon: Seoul presidency
Seoul (AFP) March 31, 2010 - North Korean leader Kim Jong-Il is highly likely to visit China soon, South Korea's presidential office said Wednesday, a move that could signal his country's return to nuclear disarmament talks.

"There is a high level of possibility that Kim Jong-Il will pay a visit to China. We are closely monitoring the situation," presidential spokeswoman Kim Eun-Hye told a briefing, declining to elaborate.

Yonhap news agency cited diplomatic sources as saying he might leave as early as Thursday or Friday and return before the annual meeting of the North's parliament on April 9.

It quoted a senior Seoul official as saying there were "indications" of an impending visit. The official noted unusual activity near the Chinese border city of Dandong and in Beijing, but gave no details.

Kim previously travelled by train to China in 2000, 2001, 2004 and 2006. He reportedly dislikes flying.

Border guards and staff at the rail station, a travel agency and a hotel in Dandong told AFP they had no news of an impending visit.

Kim's train passed through Dandong, across the Yalu border river from the North Korean city of Sinuiju, on his last trip in January 2006. That visit was shrouded in secrecy until it had ended.

YTN television aired footage showing soldiers apparently checking the bridge over the Yalu. It was unclear from the report datelined Dandong if they were North Koreans or Chinese.

Officials quoted by Yonhap said an advance North Korean team may already be in China. Pyongyang's official news agency said a military delegation held a meeting Tuesday in Beijing, but did not indicate whether they were preparing for any visit.

Analysts say any trip this year would be aimed at seeking badly needed economic aid from China, and the North in return may feel bound to return to the six-nation nuclear dialogue which Beijing hosts.

The North angrily quit the talks in April last year and vowed to restart production of weapons-grade plutonium. It carried out its second atomic weapons test the following month.

Pyongyang says it will not return to the nuclear dialogue until United Nations sanctions are lifted, and until the United States makes a commitment to hold talks on a formal peace treaty.

The North's economy is ailing and a bungled currency revaluation in November has aggravated already serious food shortages. China is its sole major ally and its most important source of food and energy.

"It is highly likely that North Korea will barter its return to the six-party talks for economic aid from China at a time when the North's economy is in bad shape following the currency revaluation," Professor Kim Yong-Hyun of Seoul's Dongguk University told AFP.

Analysts say Kim, if the trip goes ahead, may also seek Beijing's support for his attempt to install his youngest son Jong-Un as eventual successor.

The 68-year-old leader suffered a stroke in August 2008 and some reports say he also has kidney problems requiring dialysis.

"Having named his third son as successor, Kim would express hope that the Chinese leadership and his successor will maintain a good political relationship in the future," said Cheong Seong-Chang from the Sejong Institute in Seoul.

He would also urge China to speed up promised economic aid and in return would likely give a clearer pledge than previously to return to the six-party talks, Cheong said.

China stayed silent about the week-long trip in 2006. The North's official agency confirmed it only after Kim's return, saying he held talks with President Hu Jintao.

"At the talks, Kim Jong-Il expressed impressions of his visit to the central and southern parts of China where the cause of modernisation is being successfully carried out," it said.

Media reports said Kim also visited Shanghai and Guangdong, Shenzhen and Zhuhai in the south where China's economic reforms began.