South Korea vowed Wednesday to "punish the enemy" as hundreds of troops, fighter jets, tanks and attack helicopters prepared massive new drills near the heavily armed border a month after a deadly North Korean artillery attack.

Although the North backed down from its threat to retaliate over South Korean drills Monday in waters of the west coast claimed by both countries, South Korean forces have been on high alert this week, warning of surprise attacks. The North responded to a Nov. 23 artillery drill on South Korea's front-line Yeonpyeong Island with an artillery bombardment that killed four, including two civilians.

The North has made some conciliatory gestures in recent days _ telling a visiting U.S. governor that it might allow international nuclear inspections of its atomic programs _ but Seoul appears unmoved and is bracing for possible aggression.

"We will completely punish the enemy if it provokes us again like the shelling of Yeonpyeong Island," Brig. Gen. Ju Eun-sik, chief of the army's 1st armored brigade, said.

South Korea's navy began annual four-day firing and anti-submarine exercises Wednesday off the country's less-tense east coast.

The disputed western sea border has been the site of most of the Koreas' recent military skirmishes, including last month's artillery bombardment. But the east coast was used by the North as a submarine route for communist agents to infiltrate South Korea in the past.

South Korea's army and air force also planned joint firing drills Thursday near the Koreas' land border.

The training _ the 48th of its kind this year _ will be the biggest-ever wintertime joint firing exercise that South Korea's army and air force have staged, the army said in a statement. The drill will involve 800 troops, F-15K and KF-16 jet fighters, K-1 tanks, AH-1S attack helicopters and K-9 self-propelled guns, the statement said.

South Korea had planned to conduct only 47 drills of this type this year but decided to conduct one more because of continuing tension with North Korea, an army officer said on condition of anonymity citing department rules.

North Korea, meanwhile, indicated to visiting New Mexico Gov. Bill Richardson that it was prepared to consider ways to work with the South on restoring security along the border.

Richardson praised Pyongyang for refraining from retaliation and said his visit to the North provided an opening for a resumption of negotiations aimed at dismantling North Korea's nuclear program. North Korea pulled out of six-nation talks to provide Pyongyang with aid in exchange for disarmament in April 2009, but since has said it is willing to resume them.

The White House, however, rejected the idea, saying Pyongyang needed to change its "belligerent" behavior first and was not "even remotely ready" for negotiations.

In Seoul, a senior South Korean government official said the military would remain prepared for the possibility of a "surprise" attack in coming days. He spoke on condition of anonymity because of the sensitivity of the matter.

On Tuesday, a church illuminated a huge steel Christmas tree standing atop a South Korean peak that overlooks North Korean border towns, resuming a tradition condemned in Pyongyang as propaganda.

The lighting _ which needed government permission _ was 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 the North's artillery bombardment.

The North warned the tree could trigger bloodshed on the peninsula.

For decades, the rival Koreas have fought an ideological war, using leaflets, loudspeakers and radio broadcasts across the border. At the height of the propaganda, South Korea's military speakers blared messages near the border 20 hours a day, officials say.

South Korea halted the campaign about seven years ago _ including the Christmas tree lighting _ as ties between North and South warmed under an era of reconciliation. The church had sought government permission to light the tree over the years, but had been denied until this year.

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Associated Press writers Jean H. Lee, Foster Klug and Kim Kwang-tae in Seoul, Lee Jin-man in Gimpo, South Korea, and Mark S. Smith in Washington contributed to this report.