SEOUL, South Korea — A badly damaged North Korean patrol ship retreated in flames Tuesday after a skirmish with a South Korean naval vessel along their disputed western coast, South Korean officials said.

The first naval clash in seven years broke out just a week before President Barack Obama is due to visit Seoul, raising suspicions the North's communist regime is trying to rachet up tensions to gain a negotiating advantage.

There were no South Korean casualties, the country's Joint Chiefs of Staff said in a statement, and it was not immediately clear if there were any casualties on the North Korean side. Each side blamed the other for violating the sea border.

The exchange of fire occurred as U.S. officials said Obama has decided to send a special envoy to Pyongyang for rare direct talks on the communist country's nuclear weapons program. No date has been set, but the talks would be the first one-on-one negotiations since Obama took office in January.

"It was an intentional provocation by North Korea to draw attention ahead of Obama's trip," said Shin Yul, a political science professor at Seoul's Myongji University.

He also said the North was sending a message to Obama that it wants to replace the armistice agreement that ended the Korean War in 1953 with a permanent peace treaty while keeping its nuclear weapons.

Washington has consistently said that Pyongyang must abandon its nuclear arsenal for any peace treaty to be concluded. North Korea has conducted two underground nuclear tests since 2006 and is believed to have enough weaponized plutonium for half a dozen atomic weapons.

"We are sternly protesting to North Korea and urging it to prevent the recurrence of similar incidents," South Korean Rear Adm. Lee Ki-sik told reporters in Seoul.

North Korea's military issued a statement blaming South Korea for the "grave armed provocation," saying its ships had crossed into North Korean territory.

The North claimed that a group of South Korean warships opened fire but fled after the North Korean patrol boat dealt "a prompt retaliatory blow." The statement, carried on the official Korean Central News Agency, said the South should apologize.

South Korean President Lee Myung-bak, who convened an emergency security meeting, ordered the South's defense minister to strengthen military readiness.

South Korea's Joint Chiefs of Staff said in a statement that a North Korean patrol boat crossed the disputed western sea border about 11:27 a.m. (0227 GMT), drawing warning shots from a South Korean navy vessel. The North Korean boat then opened fire and the South's ship returned fire before the North's vessel sailed back toward its waters, the statement said.