SEOUL >> South Korean and U.S. troops began annual military drills Monday that rival North Korea warned could trigger a nuclear war on the divided peninsula.

Despite North Korean threats 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 a Seoul-led international investigation blamed on a North Korean torpedo; Pyongyang denies involvement. 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.

Also Monday, about 20 anti-war activists rallied near a joint forces command near Seoul, urging South Korea and the United States to immediately halt the drills and resume talks with North Korea. “Stop! Stop!” the activists chanted during the peaceful rally as a U.S. armored vehicle passed.

After weeks of high tension following its November bombardment of the South Korean island, North Korea recently pushed for dialogue with Seoul and expressed a desire to return to stalled international talks on its nuclear program. Military officers from the Koreas met earlier this month but failed to make progress.

North and South Korea are still technically at war because the 1950-53 Korean War ended with an armistice, not a peace treaty.