SEOUL, South Korea — The navies of North and South Korea clashed at sea Tuesday for the first time in seven years in what some analysts said was a provocation by the communist nation a week before President Barack Obama's visit to Seoul.

The North Korean ship retreated in flames, South Korean Prime Minister Chung Un-chan said, and the South's YTN television reported that one North Korean officer was killed and three other sailors were wounded.

The South Korean Joint Chiefs of Staff said it could not confirm the report of the North Korean casualties. There were no South Korean casualties, the military said.

Chung told lawmakers that North Korean ships violated the South's waters, although he said it was probably not intentional. He said the North Koreans may have been clamping down on Chinese fishing vessels operating in the area.

South Korean analysts, however, said North Korea was sending a clear message ahead of Obama's two-day visit starting Nov. 18.

"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.

Traveling with Obama on Air Force One, White House spokesman Robert Gibbs said the administration was aware of the clash and urged restraint on the part of North Korea.

"I would say to the North Koreans that we hope that there will be no further actions in the Yellow Sea that can be seen as an escalation," he said, referring to the body of water where the shooting took place. Koreans in both countries know it as the West Sea.

U.N. Secretary-General Ban Ki-moon is closely watching the situation and called for "maximum restraint by both parties," U.N. associate spokesman Farhan Haq said in New York. The incident shows the need to resolve all outstanding issues through dialogue and in a peaceful manner, Haq said.

The two Koreas are still technically at war and the U.S., which fought as part of U.N. forces on South Korea's side, has never had diplomatic relations with North Korea.

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.

The U.S. will send special envoy Stephen Bosworth to North Korea before year's end to try to pull Pyongyang back into international negotiations on nuclear disarmament, the State Department said.