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Lee Jin-man, Associated Press
A woman reads the messages wishing for unification of the two Koreas hanging on the barbed wire fence at the Imjingak Pavilion near the border village of Panmunjom, on the demilitarized zone (DMZ) that separates the two Koreas since the Korean War, in Paju, South Korea, Sunday, Feb. 27, 2011. North Korea threatened Sunday to attack South Korea and the United States, as the allies prepared to start annual joint military drills, maneuvers Pyongyang says are a rehearsal for an invasion.

SEOUL, South Korea — South Korean and U.S. troops began annual military drills Monday that North Korea slammed as a rehearsal for an invasion that could trigger a nuclear war on the divided peninsula.

Despite North Korea's threat to retaliate, South Korea and the United States went ahead with the drills, which are the allies' first major combined military exercises since the North shelled a front-line South Korean island in November, killing four people.

That barrage came eight months after 46 sailors were killed when a South Korean warship was sunk, an attack that an international investigation blamed on a North Korean torpedo, though Pyongyang denies it was involved. Animosity over the bloodshed drove ties between the Koreas to one of their lowest levels in decades.

About 12,800 U.S. troops and some 200,000 South Korean soldiers and reservists are to participate in the drills, which are aimed at defending South Korea and responding to any attack.

The main part of the drills, which will involve computer war games and live-firing exercises, will last 11 days, while some field training will continue until late April, according to the South Korea-U.S. joint forces command in Seoul.

The drills "are planned months in advance, and they are not connected to any current world events," the joint command said in a statement.

Hours after the exercises started, North Korea warned of a nuclear war on the peninsula.

"It's an anti-national scheme aimed at prolonging the stage of confrontation and tension to realize a plot to start a northward invasion," the North's main Rodong Sinmun newspaper said in a commentary carried by the official Korean Central News Agency. "The danger of a nuclear war on the Korean peninsula is deepening."

On Sunday, the North said that if provoked, its military would turn Seoul into a "sea of flames" and start a full-scale war with "merciless" counterattacks.

South Korean and U.S. officials have repeatedly said the drills are purely defensive.

"Denouncing these kinds of drills as an aggression and provocation won't be a help to South-North Korea relations at all," South Korean Unification Ministry spokesman Chun Hae-sung told reporters Monday.

A South Korean military official said there have been no suspicious activities by the North's military. The official spoke on condition of anonymity, citing office rules.

After weeks of high tension following its November bombardment of the island, North Korea recently pushed for dialogue with South Korea and expressed a desire to return to stalled international talks on its nuclear program.

Military officers from the two Koreas met earlier this month but failed to make progress.

The two Koreas are still technically at war because the 1950-53 Korean War ended with an armistice, not a peace treaty.