SEOUL, South Korea (AP) — South Korean and U.S. troops launched computerized military drills Monday despite North Korea warning it would retaliate with a "merciless counterblow" for the exercises Pyongyang considers rehearsal for invasion.

The 11-day drills, dubbed Ulchi Freedom Guardian, are annual war games that involve about 56,000 South Korean soldiers and 30,000 U.S. troops in South Korea and abroad, South Korea's Defense Ministry and the U.S. command in Seoul said Monday.

No field training is involved in the war games, in which alliance soldiers, mostly senior officers, sit at computers to practice how they engage in battles and hone their decision-making capabilities. The exercises are eventually designed to improve the allies' joint capability to defend the South and respond to any potential provocations, the U.S. military said in a statement last month.

Washington and Seoul engaged in joint naval drills last month off South Korea's east coast they called a show of unity after a South Korean warship was sunk in March. The allies blame a North Korea torpedo attack but Pyongyang denies involvement.

Pyongyang has for years threatened the South with destruction, though it has never followed through with an all-out military assault since the Korean War ended in 1953.

The Korean peninsula technically remains in a state of war because that conflict ended with an cease-fire, not a peace treaty. The U.S. stations about 28,500 troops in South Korea and tens of thousands more in the region.

Seoul and Washington say the routine military drills are purely defensive, while North Korea calls them preparation for an attack.

"It is another grave military provocation aimed at ... igniting a nuclear war" against North Korea, Pyongyang's main Rodong Sinmun newspaper said in a commentary carried Monday by the official Korean Central News Agency.

The country's military threatened Sunday to deal a "merciless counterblow" to the U.S. and South Korea, "the severest punishment no one has ever met in the world."

The North's military did not elaborate on how it would retaliate. South Korea's Defense Ministry said Monday it was keeping a close watch on North Korea and had spotted no suspicious activity.

Earlier this month, South Korea carried out its own naval drills near the tense western Korean sea border, and North Korea set off a barrage of artillery in response. The South is investigating whether land mines that have recently washed downriver from North Korea were deliberately floated by the North.

North Korea also seized a fishing boat with four South Korean and three Chinese crew earlier this month. The South has urged their release, but Pyongyang has not responded.

"We can prevent a war and maintain peace when we get thoroughly prepared," South Korean President Lee Myung-bak told a Cabinet meeting Monday. "We have been doing the Ulchi drills every year, but the people may feel uneasy because the drills are taking place at a time of heightened inter-Korean tension following the (ship sinking)."

On Sunday, Lee urged North Korea to abandon military provocation and make a "courageous change" toward peace, and he outlined a path for the peninsula's unification. Lee — in a speech marking the 65th anniversary of Korea's liberation from Japan's colonial rule — proposed a three-stage unification process, in which the two countries achieve peace and economic integration before becoming a "community of the Korean nation."

North Koreans also marked Liberation Day by paying respect to a huge statue of the late founder of their country, Kim Il Sung. Streams of soldiers, ruling Workers' Party officials and ordinary citizens offered bouquets of flowers and bowed deeply before Kim's statue, according to the North's state media.