The boat retreated to its side about two hours and 40 minutes after it had intruded across the disputed border, the Joint Chiefs of Staff said in a statement.

A South Korean navy boat fired 10 warning shots from a machine gun after broadcasting three verbal warnings, it said. No one was reported hurt.

The North's vessel appeared to have been unable to move for a time because the tide was low, the military said.

The intrusion came at a sensitive time, with the South's military and police on high alert before next week's Group of 20 summit in Seoul.

"The intrusion came as our troops have been placed on top alert off the west coast," a defence ministry spokesman said.

However he said there were no unusual naval activities by the North along the sea border, which was the scene of deadly naval battles in 1999 and 2002 and a firefight last November which left a North Korean boat in flames.

In March a South Korean corvette sank in the area with the loss of 46 lives. Cross-border tensions have been high since Seoul, citing a multinational investigation, accused Pyongyang of torpedoing the ship. It denies the charge.

North and South Korean troops exchanged fire briefly last Friday across their tense land border, without apparent casualties.

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