SEOUL (Reuters) – South Korean President Lee Myung-bak said yesterday the sinking of a warship in which 46 sailors were killed was no accident but he stopped short of blaming North Korea and made clear he was not about to order a revenge strike.

Senior government officials privately make little secret of their belief that the reclusive neighbour deliberately torpedoed the 1,200-tonne Cheonan on March 26 near their disputed border in retaliation for a naval firefight last year in which the South bested the North.

"One sure fact is that the Cheonan did not sink because of a simple accident," Lee Myung-bak told a meeting of his top military commanders in a nationally televised speech. The issue casts something of a cloud over this week's visit by North Korea's iron ruler Kim Jong-il to China, the only major power he can count on to prop up the isolated state's broken economy and his own family's more than 60-year grip on power. International investigators are still examining the reason for the explosion that sank the ship but the government has already said it came from outside the vessel, which leaves little other explanation than a North Korean attack. But Lee avoided directly accusing Pyongyang and also underscored that this was not something to be resolved alone by Seoul.

"As soon as the incident occured I felt immediately that, along with relations between South and North Korea, that it was an international problem."

"When the international joint investigation group reveals the reason for the accident, the world will know. After the reason is found, there will be firm and definite action taken."

His government has already said if the North was the culprit it would raise the matter with the United Nations Security Council. That could put China, as one of the five permanent members of the council, in something of a bind.

Analysts say Beijing is willing to support Kim Jong-il rather than risk instability on its border from the violent implosion of the impoverished state. But it did back international sanctions after the North's last nuclear test in 2009.

The issue is also fraught with risk for President Lee. A strike against the North would raise the possibility of a broader conflict which the South would almost certainly win, with the help of US military in the region, but in the process frighten away investors and damage Asia's fourth biggest economy. He also needs to be seen at home as successfully handling the aftermath of the attack ahead of local elections in June, which could determine whether he serves the rest of his single five-year term as a lame-duck leader, unable to push through his economic and social reform agenda.