SEOUL, South Korea  — South Korea vowed Wednesday to "completely punish" North Korea if it attacks again, and mobilized hundreds of troops, tanks and helicopters for a massive military exercise prompted by high tensions on the peninsula.

The firing drills planned for Thursday near the Koreas’ heavily armed land border signaled that South Korea is willing to risk further escalating tensions with North Korea, which shelled a southern island off the western coast on Nov. 23 and stirred up a war-like atmosphere.

The attack, which killed four people, was portrayed by Pyongyang as a retaliation for southern military exercises on Yeonpyeong island that day.

South Korea has conducted 47 similar military drills this year, and it scheduled one more exercise for Thursday in response to the North Korean attack, an army officer said on condition of anonymity citing department rules. Thursday’s drill will be the biggest-ever wintertime joint firing exercise that South Korea’s army and air force have staged, an army statement said.

"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 separately.

South Korean forces are on high alert even though the North backed down from its threat to again retaliate over a separate firing drill the South held Monday on Yeonpyeong in disputed western waters.

The two Koreas remain technically at war since their 1950s conflict ended in a cease-fire, not a peace treaty. The past month’s military tension, however, has been the worst in several years.

The North has made conciliatory remarks in recent days — telling a visiting U.S. governor it might allow international inspections of its nuclear programs — but Seoul is mindful of past surprise attacks and still is bracing for possible aggression.

South Korea’s navy also began annual four-day firing and anti-submarine exercises Wednesday off the country’s eastern coast. That area has been less tense recently but in the past, the North has used eastern waters as a submarine route for communist agents to infiltrate South Korea.

The Koreas’ recent military skirmishes, including last month’s artillery bombardment, have been in the tense western waters, where Pyongyang does not recognize the U.N.-drawn border.

Thursday’s air force and army drills will involve 800 troops, F-15K and KF-16 jet fighters, K-1 tanks, AH-1S attack helicopters and K-9 self-propelled guns. They will take place in Pocheon, about 30 miles (45 kilometers) north of Seoul and about 21 miles (33 kilometers) south of the North Korean border.

Seoul has relocated more artillery on Yeonpyeong island following last month’s shelling and plans to deploy Israeli-made Spike missiles there soon, Yonhap news agency reported, citing an unidentified military official. The Joint Chiefs of Staff declined to confirm the report.

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.

Separately, about 200 South Koreans attended a candlelight vigil Wednesday evening for the four South Koreans killed by North Korea’s attack on the Yeonpyeong.

They observed a moment of silence and placed flowers on a makeshift mourning site in central Seoul. "We, the survivors, should remember their sacrifice and make efforts to ensure that their sacrifice will not be in vain," said Choi Hong-jae, a 42-year-old executive.

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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.