South Korea Wednesday sent two more warships to guard its border after a naval clash left a North Korean patrol boat in flames, as Washington confirmed it would send an envoy for nuclear talks in Pyongyang. Military sources told selfnews the 1,800-ton patrol boats would 'reinforce vigilance' along the disputed Yellow Sea border, where the navies of the two Koreas clashed Tuesday for the first time in seven years. The defence ministry said it could not comment on operational matters. Sources quoted by local media said one North Korean sailor was killed and three wounded in Tuesday's exchange of fire, 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 the State Department announced Tuesday it was accepting an invitation to send an envoy to Pyongyang to try to bring the North back to six-nation nuclear disarmament talks. It said the US envoy to North Korea, Stephen Bosworth, would probably visit before year-end. Secretary of State Hillary Clinton, speaking in Singapore Wednesday, said the trip would be unaffected by 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.' Seoul's 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 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 selfnews. 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. A legislator quoted South Korean intelligence authorities as telling a parliamentary committee that they believe the attack was intended to serve 'a limited purpose'. 'There was also a view that it was intended to test how our military would respond in the event of an NLL (border) violation,' the lawmaker told Yonhap news agency. 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 the six-party 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 its 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. 