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 two Koreas' navies.

A South Korean Marine looks at the North Korean village, Kaepoong, at an observation post in Ganghwa, South Korea, 11 Nov 2009A spokesman for South Korea's joint chiefs of staff says the South's army, navy and air forces are closely monitoring the activities of the North's military.  

But South Korean officials say so far they have not noticed any unusual troop movements by the North.

Wednesday Seoul sent two additional warships to guard the maritime border in the Yellow Sea.

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

South Korea's military says that during Tuesday's brief skirmish, one of its warships first fired warning shots at a North Korean boat before the clash ensued. The South 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.

Each side has asked the other to apologize for the incident.

Following the clash, Washington and the United Nations called for restraint in the Yellow Sea.  

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.

Last month, North Korea's navy accused South Korea of sending warships across the maritime border to stir up tensions and warned that such actions could trigger armed clashes.

North and South Korea fought two deadly naval battles in the Yellow Sea in the past decade. 

Some information for this report was provided by AFP.