Seoul: South Korea's military on Thursday held a live-fire drill involving tanks, artillery and jet fighters, in a major show of strength staged exactly a month after North Korea's deadly attack on a border island.

Washington expressed support for the live-fire exercise by its ally, the second this week, but Pyongyang criticised the South's 'puppet warmongers'.

The exercise at Pocheon, 30 kilometres (20 miles) south of the tense land border with North Korea, began at 2:43 pm (0543 GMT), a defence ministry spokesman said.

The exercise, the largest ground-air joint fire drill this year, ended after about 40 minutes, according to a pool report from a firing range at Pocheon.

Some 800 troops took part along with 30 K-1 tanks, 11 K-200 armoured personnel carriers, two F-15K jets, four KF-16 jets, 36 K-9 artillery pieces, three multiple long-range rockets, four 500MD helicopters, three AH-1S Cobra helicopters, and other equipment.

The navy is also conducting a four-day exercise off the east coast, which began on Wednesday.

The South says its drills are defensive. But tensions have been high on the peninsula since the North shelled a South Korean island near the contested western sea border on November 23.

The North said its shelling was in response to the South's live-fire drill on Yeonpyeong island. The South said it had been staging such artillery exercises for 37 years and the North was seeking a pretext to attack.

Seoul staged a repeat drill on the same island on Monday, backed up by jet fighters and warships, but the North did not follow through with threats to hit back.

Some analysts said Seoul's show of force deterred the North. Others said the hardline regime had been told by close ally China to exercise restraint before a visit to Washington by President Hu Jintao starting on January 19.

The military invited students and other civilians to watch the exercise.

"We are facing a crisis because of North Korea, so I came to see this air and ground operation," Kim Tae-Dong, a 70-year-old Internet businessman, told a pool reporter.

"I want to feel and see the level of South Korea's armed forces," Kim said.

"Another North Korean provocation will happen. We should prepare our military perfectly for that."

Analysts agreed, saying that while Pyongyang had shown restraint this time it was likely just biding its time for another military strike.

"It's not a question of whether there will be another provocation, but when," said Peter Beck, a North Korea expert with the Washington-based Council on Foreign Relations.

The North's official news agency said the South's claims that the drills are routine were an attempt "to conceal the provocative and offensive nature of the exercises".

The wording was relatively mild. In another sign that tensions are easing, the South's Joint Chiefs of Staff said it had lowered a military alert issued for frontline areas before and during Monday's drill.

Vulcan artillery vehicles fired into a wide valley with numbers carved on hills below to launch the show of strength.

Tanks raced along roads, firing as they went. A hillside blossomed smoke as artillery and rockets opened up.

Hovering helicopters fired rockets at targets, and F-15 aircraft dropped bombs into the valley, sending up huge plumes of smoke.

The United States, which has 28,500 troops based in the South, earlier warned North Korea there was no reason for it to respond to the latest drills.

White House spokesman Robert Gibbs said the manoeuvres had been announced well in advance and were transparent and defensive, and "should in no way engender a response from the North Koreans".

The South's military was heavily criticised for a perceived feeble response to last month's attack. It has been stressing its battle-readiness and determination to hit back harder next time, using air power.

President Lee Myung-Bak on Thursday visited a frontline army unit and 'confirmed troops were fully on guard against possible enemy provocations', a statement from his office said.