SEOUL, South Korea - In a move that many fear will provoke an already testy North Korea, Seoul officials yesterday announced the start of massive new live-fire drills involving troops, tanks, fighter jets and anti-aircraft guns - as well as six naval ships and Lynx anti-submarine helicopters.

The maneuvers, which will run through tomorrow, were the latest in an escalating round of clashes and military exercises involving the two Koreas and came as the South's armed forces remained on high alert for Pyongyang to retaliate for a drill held earlier this week.

North Korea had vowed to strike with lethal force if the South fired artillery shells off remote Yeonpyeong Island, the target of a Nov. 23 artillery bombardment by Northern forces that killed four people.

But after the South's 90-minute drill off the island Monday, North Korea dismissed the exercise as "not worth reacting" to.

Still, South Korean officials say the military will remain extra vigilant until there are concrete signs that tensions between the two neighbors are easing. "We will completely punish the enemy if it provokes us again like the shelling of Yeonpyeong Island," said Brig. Gen. Ju Eun-sik.

For weeks, tensions have remained high on the Korean peninsula, with the U.N. Security Council holding an emergency session this week to devise ways to calm nerves.

South Korea's land drills, scheduled to be held today about 30 miles north of Seoul, will reportedly involve the largest number of personnel in a peacetime exercise. The naval exercises will take place off the nation's less-tense eastern coast, not the disputed western sea border - the site of the Yeonpyeong attack.

White House spokesman Robert Gibbs discounted the possibility that the drills would lead to North Korean retaliation. "Exercises that have been announced well in advance, that are transparent, that are defensive in nature, should in no way engender a response from the North Koreans," he said.