Sea fog has delayed the start of a live-fire drill on a South Korean border island, the defence ministry said, after announcing the exercise would go ahead Monday despite North Korean threats.

Spokesman Kim Min-Seok, announcing the delay, said the exercise on Yeonpyeong island would last less than two hours.

"The military is ready to start the drill anytime today if the weather improves," another ministry spokesman told AFP.

A similar artillery exercise by marines on Yeonpyeong on November 23 was answered by a North Korean bombardment which killed four people including civilians there and damaged dozens of homes.

The North has threatened an even deadlier response this time, saying the shells fired in such drills fall into its territorial waters. It refuses to recognise the Yellow Sea borderline.