**South Korean President Lee Myung-bak has visited an area off the North Korean coast where a South Korean warship sank after an explosion.**Forty-six sailors are missing after the navy ship went down on Friday near the disputed inter-Korean border.
Officials say they believe there could be survivors trapped in cabins and have injected oxygen into parts of the hull.
The cause of the explosion is not yet clear, but a senior official has said it could have been a North Korean mine.
Early on Tuesday, Mr Lee ordered the military onto alert, saying: “Since the sinking took place at the front line, the military should thoroughly prepare for any move by North Korea.”
He then flew to Baengnyeong Island, 16km (10 miles) from the North Korean coast, to see the rescue efforts.
Swift currents
Fifty-eight crew members were rescued when the Cheonan went down late on Friday.
So far, swift currents have prevented divers from gaining access to the ship, which split in two. Divers who knocked on the hull on Monday received no response.
Teams used a hose to inject oxygen into the stern via a crack and officials said they expected to make more progress on Tuesday.
More than a dozen South Korean ships are involved in the rescue effort, plus a US vessel.
On Monday, Defence Minister Kim Tae-young said the blast could have been caused by a mine laid by the North during the 1950-53 Korean War.
He said it could also have been a mine that the communist state intentionally sent floating towards the South Korean vessel.
But an internal malfunction has not been ruled out and military officials said establishing the cause of the blast might have to wait until the ship is salvaged.
Pyongyang has made no official comment on the incident.
It does not accept the maritime border, known as the Northern Limit Line, which was drawn unilaterally by the US-led United Nations Command at the end of the Korean War.
The area has been the scene of deadly clashes between the navies of the two Koreas in the past. This article is from the BBC News website. © British Broadcasting Corporation, The BBC is not responsible for the content of external internet sites.