North Korea warned South Korea of a possible naval clash Thursday, accusing Seoul of sending warships into its waters around their disputed western sea border.

The South's "reckless military provocations" have created "such a serious situation that a naval clash may break out between the two sides in these waters," the North's navy said in a statement carried by the country's official Korean Central News Agency.

North Korea has often issued similar warnings before, as it does not recognize the western sea border. The communist nation claims that the United Nations unilaterally drew the line at the end of the 1950-53 Korean War, and that it should be redrawn further south.

The dispute led to two bloody naval skirmishes in 1999 and in 2002.

But the latest warning came as relations between the two Koreas showed signs of improvement with the North taking a series of conciliatory steps like freeing detained South Koreans and pledging to resume stalled joint projects.

On Wednesday, the North offered a rare apology to the South for releasing a massive amount of water from a dam that sparked flooding blamed for six South Korean deaths.

Ties between the two sides had badly frayed as North Korea cut off reconciliation talks and suspended joint projects in anger over the hard-line policy that the South's conservative President Lee Myung-bak has taken toward the North since taking office last year.

The two Koreas fought the 1950-53 Korean War, which ended in a truce, not a peace treaty, which means that the sides are still technically at war.