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South Korea lit a 30-metre steel Christmas tree in Gimpo, near the demilitarized zone, on Tuesday, for the first time in seven years. ((Lee Jin-man/Associated Press))

South Korea lit a massive steel Christmas tree overlooking its armed border with the North on Tuesday, marking a pointed return to a tradition that Pyongyang has condemned as propaganda.

Troops stood guard and a choir sang carols as the 30-metre-tall tree was lit for the first time in seven years.

The provocative ceremony — which must receive government permission — was also a sign that President Lee Myung-bak's administration is serious about countering the North's aggression with measures of its own in the wake of an artillery attack that killed four South Koreans last month.

Earlier Tuesday, a South Korean destroyer prowled the sea and fighter jets screamed across the skies in preparation for possible North Korean attacks. On Monday, the South staged provocative artillery drills on Yeonpyeong Island, where the attacks occurred.

North Korea has said it would not retaliate for the exercises off Yeonpyeong — reversing its earlier threats.

A senior South Korean government official, however, said that the lack of response so far does not mean Pyongyang is backing down. Speaking on condition of anonymity, he noted that North Korea thrives on "surprise" attacks.

Pyongyang considers the waters around the island — a tiny enclave of fishing communities and military bases within sight of North Korean shores — its territory. Similar drills last month triggered an artillery barrage that killed four South Koreans, in the first attack targeting civilian areas since the 1950-53 Korean War.

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South Korean marines patrol on Yeonpyeong Island on Tuesday as the South prepares for possible attacks from North Korea. ((Reuters))

North Korean officials have agreed to let UN atomic inspectors visit its main nuclear complex to make sure the facility is not producing enriched uranium for a nuclear bomb, New Mexico Gov. Bill Richardson said Tuesday while on an unofficial visit to the communist state.

Richardson said that could provide an opening for a resumption of negotiations aimed at dismantling the country's nuclear program.

North Korea pulled out of six-nation talks to provide Pyongyang with aid in exchange for disarmament in April 2009. It has since said it is willing to resume them.

In Seoul, meanwhile, top officials defended South Korea's decision to carry out more drills despite calls in some quarters for restraint amid fears of all-out war, and said the military was prepared for any future North Korean aggression.

Seoul's decision to push ahead with the routine drills in the face of North Korean threats of nuclear war and pressure from China and Russia indicates a new willingness by President Lee Myung-bak's government to use provocations of its own to counter North Korean aggression. Seoul has already cut aid to the impoverished North and refused to participate in moneymaking joint tourism projects in North Korea.

South threatens airstrikes

Accused of acting too slowly and too weakly last month, Lee has threatened airstrikes if hit again and ordered more troops to front-line islands. On Tuesday, he gathered his national security advisers for strategic talks.

"When it provokes, we will firmly punish North Korea," Defence Minister Kim Kwan-jin told lawmakers before leaving for the security meeting Tuesday.

Pyongyang denounced the 90-minute exercise as a "reckless military provocation" but held its fire.

The Korean People's Army, however, showed no signs of pulling back.

North Korea deployed SA-2 ground-to-air missile and ground-to-ship missiles in the west — where the Koreas dispute their sea border — that are poised to fire artillery, South Korea's Yonhap news agency reported, citing an unidentified military official.

South Korean fighter jets continued patrolling the skies and an Aegis-equipped destroyer was ready to counter any possible provocation, the Defence Ministry said.