Tanks, armoured personnel carriers, artillery pieces, helicopters and jets took part in the drill at the Pocheon range, 30 kilometres (20 miles) south of the border, the defence ministry said.

The show of force came a day after the North's military threatened to fire back in retaliation for a planned launch of leaflets by anti-Pyongyang activists near the disputed Yellow Sea border.

South Korea is also holding a week-long sea exercise that began on Monday to mark the first anniversary of its warship sinking on March 26 last year.

The South accused the North of torpedoing the ship with the loss of 46 lives. Pyongyang denies the charge.

The sea exercise reportedly includes three days of training by South Korean warships and aircraft from Friday, simulating attacks by North Korean submarines, ships and jet fighters.

Massive naval firing training will be held on Saturday at three locations off the peninsula.

The South, which has remained technically at war with its nuclear-armed neighbour since their conflict 60 years ago, says its drills are defensive.

But tensions between the two nations have risen since the North shelled a border island on November 23, killing four people.

This year South Korea has been staging a series of drills alone or together with US troops to test its battle readiness.

US and South Korean troops completed computerised joint war games on March 10, but joint field training will continue throughout March and April.

The North's military warned Wednesday that it would open fire if defectors from the communist country launch propaganda leaflets from Baengnyeong island, the closest land to the sea border.

The defectors plan to launch leaflets on Friday or Saturday that would contain news of Arab uprisings and call for the overthrow of the North's regime.