SEOUL, South Korea (AP) - North Korea threatened Thursday to punish South Korea after their first naval skirmish in seven years, as Seoul expressed confidence it could deter any attack from its communist neighbor.

The two Koreas clashed in waters off their western coast Tuesday with each side accusing the other of violating the disputed sea border and firing first. The fighting came ahead of a planned visit to South Korea next week by President Barack Obama.

On Thursday, the North's main Rodong Sinmun newspaper said in a commentary that it will not tolerate what it claimed was South Korea's aggression in its waters.

"Our unchanged principle is no forgiveness and merciless punishment for warmongers who infringe upon our republic's dignity and sovereignty," said the commentary, carried by the official Korean Central News Agency. It didn't specify how the North would punish the South.

Another state newspaper, Minju Joson, also warned that South Korea would face "costly consequences." It said the clash stemmed from a plot by the South to disrupt direct talks that are planned between Pyongyang and Washington by inspiring anti-North Korea sentiment among American officials.

Analysts agree that the skirmish was likely linked to Obama's visit - though they have suggested that the North may have been looking to improve its bargaining position with the U.S. by stoking tensions.

Obama, due to arrive in Seoul on Nov. 18 for a regional meeting, plans to send a senior envoy to Pyongyang by year's end for the first direct talks between the wartime foes during his administration. U.S. Secretary of State Hillary Rodham Clinton said in Singapore on Wednesday that the fight would not scuttle a planned visit to Pyongyang by special envoy Stephen Bosworth.

Bosworth's trip is aimed at persuading communist North Korea to return to six-nation nuclear disarmament negotiations, which Pyongyang walked away from earlier this year.

South Korean officials shrugged off the North's threats, saying they were ready to deter any aggression.

"We will resolutely safeguard" the Northern Limit Line, a de facto western sea border drawn up by the U.N. command at the end of the 1950-53 Korean War, a Defense Ministry official said. The North has long insisted the line be redrawn farther south.

An officer with the Joint Chiefs of Staff also reiterated that the skirmish broke out as the North Korean ship opened fire after violating the border and ignoring warnings shots from the South Korean ship.

Both officials spoke on condition of anonymity citing department policy.

The mass-circulation Dong-a Ilbo newspaper reported Thursday that South Korean ships fired a total of about 4,000 rounds at the North Korean vessel, inflicting so much damage that it had to be towed by another North Korean ship to a nearby naval base.

One senior South Korean officer told The Associated Press that a North Korean was killed and three others wounded. He spoke on condition of anonymity because the matter involved intelligence.

A South Korean ship was lightly damaged, and there were no casualties on their side, officials said.

Yang Moo-jin, a professor at Seoul's University of North Korean Studies, downplayed the seriousness of the North's threats, saying Pyongyang was unlikely to take further military action because it appeared to be using the skirmish to get Washington's attention. He said it was in their interest to "show the Korean peninsula is still unstable" ahead of anticipated direct talks.

Following the skirmish, South Korea's 680,000-member military went on high alert to cope with possible retaliation. South Korean media reported the country has deployed up to four destroyers and warships near the sea border - the scene of two bloody fights in 1999 and 2002.

South Korea's military said there has been no sign of suspicious activities from North Korean troops, but news reports said the North has also placed its 1.2 million-strong army on high alert.

Defense Minister Kim Tae-young told the National Assembly on Tuesday that he believed the North may take retaliatory actions, saying President Lee Myung-bak "also has such concerns."

The two Koreas have remained technically at war since the Korean War ended with an armistice, not a peace treaty. The U.S., which has never had diplomatic relations with North Korea, stations 28,500 troops in South Korea to deter potential North Korean aggression.

Associated Press writers Kwang-tae Kim in Seoul and Matthew Lee in Singapore contributed to this report.