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South Korea's military is on high alert for possible retaliatory moves after a North Korean patrol boat was set ablaze Tuesday during a clash between the countries' navies.

Seoul sent two additional warships to guard the maritime border in the Yellow Sea on Wednesday. But South Korean officials say they have not noticed any unusual troop movements by North Korea.

The United States and the United Nations are calling for restraint in the Yellow Sea.  

U.S. Secretary of State Hillary Clinton said Wednesday that Washington still plans to send a high-ranking diplomat to North Korea, despite the naval clash.

The U.S. State Department says special envoy Stephen Bosworth likely will visit Pyongyang before the end of the year. He will try to revive the six-nation negotiations on North Korea's nuclear weapons program.   

The naval skirmish this week threatened to complicate the talks involving both Koreas, the U.S., China, Russia and Japan.

South Korea's military says that during Tuesday's brief skirmish, one of its warships fired warning shots at a North Korean boat before the clash ensued. Seoul says the North's boat crossed its disputed maritime border off the western coast of the Korean peninsula.

North Korea's official (KCNA) news agency says its patrol boat was on a mission to confirm an unidentified object on its side of the border.

Kim Sung-hwan, a top presidential aide responsible for South Korean security affairs, says Pyongyang may take its time and wait for the best opportunity to retaliate.

U.S. President Barack Obama is due to make a two-day visit to South Korea next week as part of his tour of Asia.  One issue high on his agenda is international efforts to dismantle North Korea's nuclear weapons program.