South Korean and US troops have begun annual military drills, which North Korea warns could trigger a nuclear war on the divided peninsula.
Despite the North Korean threats to retaliate, South Korea and the United States went ahead with their first major combined military exercises since the North shelled a frontline South Korean island in November, killing four people.
About 12,800 US troops and some 200,000 South Korean soldiers and reservists are taking part 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-US 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 realise 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.
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" counter-attacks.
South Korean and US officials have repeatedly said the drills are purely defensive.
"It's very difficult to see improved South-North Korean ties before North Korea changes its attitude," South Korean Unification Minister Hyun In-taek said, according to his office.