SEOUL - A NORTH Korean navy patrol boat on Thursday crossed into South Korean waters and stayed almost one hour before retreating after repeated warnings, Seoul military officials said.

The boat crossed the Yellow Sea border in the early afternoon but returned to its own side about 50 minutes later, the Joint Chiefs of Staff said in a statement.

Tensions are high along the disputed maritime frontier known as the Northern Limit Line (NLL) since the North renounced the armistice which ended the 1950-1953 war on the peninsula and threatened attacks on the South. It was protesting at the South's decision to join a US-led anti-proliferation exercise following the North's nuclear test on May 25.

The North Korean boat intruded 1.6 kilometres into the South Korean side, a spokesman for the Joint Chiefs told AFP. He said it apparently was chasing Chinese boats that were operating illegally in the rich crab fishing area, but did not exclude the possibility it was a planned intrusion to raise tensions further.

South Korea radioed a warning message to the boat once before and twice after it trespassed, he said, adding it was the third time this year the North has crossed the maritime border.

'Our patrol boats sailed up to stop the intrusion,' the spokesman said, without specifying how close the two sides came to each other.

South Korean and US troops in the peninsula have gone on heightened alert since the North said it was no longer observing the truce.

Seoul has deployed a high-speed patrol boat armed with ship-to-ship missiles to the area, the scene of bloody naval clashes in 1999 and 2002.

'Our side was cautious to prevent another clash,' the spokesman said About 70 of some 90 Chinese fishing boats withdrew overnight from the area, Yonhap news agency said.

The NLL was drawn unilaterally by United Nations forces after the war. The North wants it drawn further to the south. -- AFP