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

The maneuvers, which will run through Friday, 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 Thursday 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.

In Washington, Col. Dave Lapan, a Pentagon spokesman, said: "These are standard, routine training exercises on established ranges."

White House spokesman Robert Gibbs discounted the possibility that the drills would lead to a 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.

North Korea also has visibly increased its military presence in recent weeks, officials say. The North has reportedly deployed more surface-to-ship and surface-to-air missiles along its west coast while keeping fighter jets at the ready.

Pyongyang has also made some conciliatory moves, reportedly saying it might allow international nuclear inspectors access to its atomic programs for the first time since last year.

But Seoul appears unmoved and is bracing for possible aggression.

South Korean President Lee Myung Bak has been pressured to send the North a decisive message since the deadly torpedoing of a Southern warship in March that has been blamed on Pyongyang. The nation's defense minister stepped down last month, and his replacement has vowed a new aggressive stance.