SEOUL: South Korea's military was on alert Wednesday for any retaliatory moves after a North Korean patrol boat was set ablaze in a naval clash, as Washington warned Pyongyang against escalating tensions.

Defence Minister Kim Tae-Young ordered army, navy and air force commanders in charge of border areas to step up surveillance and respond immediately to any provocation, a spokesman for Seoul's Joint Chiefs of Staff said.

Tuesday's clash near the disputed Yellow Sea border raised tensions just over a week before US President Barack Obama arrives in Seoul as part of an Asian tour.

White House spokesman Robert Gibbs called on the North to avoid any further actions "that could be seen as an escalation". US warns North Korea after naval clash

But Washington also announced it is accepting an invitation to send an envoy to Pyongyang to try to bring the communist state back to a nuclear disarmament talks.

The State Department said the US envoy to North Korea, Stephen Bosworth, would visit at an appropriate time, probably before year-end.

Military sources quoted by newspapers in Seoul said one North Korean sailor was killed and three wounded in the brief but fierce exchange of fire on Tuesday. The defence ministry said it could not confirm the figure. No South Koreans were injured.

Analysts were undecided whether the North deliberately ordered a patrol boat across the border to raise regional tensions before Obama's visit, or whether a lone commander was at fault.

Seoul said the North's boat ignored five warnings to turn back and then opened fire at a South Korean boat which had fired a warning shot. One or two South Korean boats then returned fire.

North Korea's military, however, told its South Korean counterpart to apologise for a "grave armed provocation" and said Seoul's ships had opened fire while its craft was north of the border.

The disputed sea frontier has always been a potential flashpoint and was the scene of bloody naval battles in 1999 and 2002.

President Lee Myung-Bak called Tuesday for calm to prevent any escalation. His Prime Minister Chung Un-Chan described the clash, which follows recent peace overtures from the North, as unplanned.

But Lee is concerned about a possible retaliatory move by the North, Defence Minister Kim Tae-Young told parliament Tuesday.

The top presidential aide for security affairs, Kim Sung-Hwan, told Yonhap news agency Wednesday there were no signs of preparations for any revenge strike.

"But the North may take its time, waiting for the best chance for retaliation. We're taking countermeasures," he said.

Cross-border tensions have been high for more than a year and the North has also angered the international community with missile test-launches and a second atomic weapons test. The United Nations tightened sanctions in response. Chronology: Clashes on Korean peninsula

Since August the North has put out peace feelers to Washington and Seoul, inviting Bosworth for talks to improve "hostile relations."

Pyongyang says it is ready to rejoin six-nation disarmament talks which it quit in April if the US discussions go well.

Washington stresses that the bilateral talks are intended only to bring Pyongyang back to the six-party forum, which also includes South Korea, Japan, China and Russia.

"This is not the beginning of a bilateral dialogue that is separate from, you know, the six-party process," said State Department spokesman P.J. Crowley.