The North Korean vessels, which included a towing boat and two other small vessels, crossed the northern limit line (NLL), the de-facto maritime border, and intruded 0.5 miles (0.8 kilometres) into southern waters, the Joint Chief of Staff said.

A South Korean patrol boat broadcast warnings immediately after the North Korean vessel crossed the line at 2:30 p.m. (05:30 GMT).

Five minutes later, the South Korean vessel fired five rounds from a machine gun and when the North Korean boats failed to respond, fired a further four rounds from a 40-milimeter-gun.

The North Korean boats then headed north, the Joint Chiefs said in a statement.

"Because of poor visibility and long distance, we could not discern what kind of ships they were," an official at the Joint Chiefs told AFP.

It was the third time this year that South Korean navy vessels had fired warning shots to chase North Korean ships from disputed waters in the Yellow Sea.

South Korean patrol boats fired shots twice in June to drive North Korean fishing boats from rich fishing grounds claimed by both Koreas.

WAR.WIRE