North and South Korean navies have engaged in an exchange of fire off their west coast, leaving a North Korean naval boat severely damaged.

The clash took place at 11.28 am (0228 GMT) near Daechong island when a South Korean warship shot at a North's navy boat that crossed the disputed western sea border on Tuesday morning.

A North Korean patrol boat crossed the border and sailed south for about 0.7 miles (1.1 kilometers), Seoul's Joint Chiefs of Staff (JCS) said in a statement.

It said the North's craft held its course despite several warning signals from a South Korean high-speed naval boat, which resorted to firing warning shots.

But "the North's side opened fire, directly aiming at our ship. Then our ship responded by firing back, forcing the North Korean boat to return to the north," the statement added.

"There were no casualties on our side. We are on the lookout for any further provocations by the North."

A JCS spokesman said the North Korean boat was considerably damaged by heavy cannon fire, while a navy source said the boat was half-destroyed when it was forced to sail back to the North.

South Korea's YTN television quoted military sources as saying the North's boat crossed the border while trying to stop illegal fishing by Chinese boats in the rich crab-fishing grounds.

The JCS also held an emergency session following the incident to decide whether or not the border crossing was a deliberate provocation.

In October, Pyongyang's naval force accused South Korea of sending warships across the border to stir tensions, warning the 'reckless military provocations' could trigger armed clashes.

The disputed border in the Yellow Sea, known as the Northern Limit Line (NLL), was the scene of fierce naval conflicts in 1999 and 2002.

The NLL, drawn up unilaterally by United Nations at the end of the Korean War in 1953, has been rejected by Pyongyang, that wants it drawn further to the south.

MRS/SC/DT