SEOUL - South Korea Wednesday sent two more warships to guard its disputed Yellow Sea border with North Korea, military sources said, a day after a naval clash raised tensions on the peninsula.

The sources told the Agence France-Presse the 1,800-ton patrol boats would "reinforce vigilance" along the sea border, where Seoul said a North Korean patrol boat was set ablaze Tuesday in an exchange of fire.

The defence ministry said it could not comment on operational matters.

Military sources quoted by newspapers said one North Korean sailor was killed and three wounded in the brief but fierce exchange of fire Tuesday, 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".

But Washington announced it was accepting an invitation to send an envoy to Pyongyang to try to bring the North back to nuclear disarmament talks. The State Department said the US envoy to North Korea, Stephen Bosworth, would probably visit before year-end.

Secretary of State Hillary Clinton, speaking in Singapore, confirmed the trip would go ahead despite the naval clash.

"We are obviously hoping that the situation does not escalate and we are encouraged by the calm reaction that has been present up until now," she told reporters at Asia-Pacific talks.

"But this does not in any way affect our decision to send Ambassador Bosworth. We think that is an important step that stands on its own."

Defence Minister Kim Tae-Young ordered army, navy and air force commanders along the border with the communist North to step up surveillance and respond immediately to any provocation.

However the government does not want the clash to damage cross-border relations, said presidential spokeswoman Kim Eun-Hye. After months of bellicose moves, the North has recently made overtures to Seoul and Washington.

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

Officials could not confirm any North Korean casualties but said no South Koreans were hurt, although one South Korean boat was hit 15 times.

North Korea's military blamed the South for a "grave armed provocation". It said Seoul's ships had opened fire while its craft was north of the border, which was the scene of bloody battles in 1999 and 2002.

Some analysts said the North's leadership may be strengthening its bargaining hand in the upcoming talks with the United States by heightening tensions.

"The intrusion might have been pre-planned to raise tensions," Baek Seung-Joo of the Korea Institute for Defence Analyses told AFP.

But Baek said retaliation from the North for damage to its boat was relatively unlikely because of the South's superior naval firepower and because of the upcoming dialogue with the United States.

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

"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, a walkout from six-party nuclear disarmament talks and a second atomic weapons test.

The United Nations tightened sanctions in response.

The North now says it is ready to rejoin the six-nation talks if the US discussions go well. Washington stresses the talks are intended only to bring Pyongyang back to the six-party forum, which also includes South Korea, Japan, China and Russia.

About 300 activists rallied in Seoul, torching portraits of North Korean leader Kim Jong-Il in protest at the naval clash.