South Korea-US drills begin despite North Korean threat
US and South Korean troops are staging major annual land, sea and air drills, prompting North Korea to threaten "all-out war" on the peninsula.
Pyongyang warned it would turn Seoul into a "sea of fire" if provoked. The US and South Korean joint command insist the drills are purely defensive.
Relations have been fraught since the North shelled a South Korean island in November, killing four people.
Military talks aimed at easing tensions between the two broke down recently.
The 11-day exercises involve 200,000 South Korean troops and nearly 13,000 Americans - most of whom are not based in the country.
The training drills are a regular rehearsal for emergency deployments of US forces in the event of a sudden attack on South Korea.
The drills "are planned months in advance, and they are not connected to any current world events", the joint command said in a statement.
Every year Pyongyang denounces the exercises, saying they are actually a pretext for an American invasion of the North to topple the communist government.
Just hours after the exercises started, a commentary in North Korea's main newspaper warned "the danger of a nuclear war on the Korean peninsula is deepening".
Earlier, the North threatened to turn the South's capital, Seoul, which sits less than 48km (30 miles) from the border and so well within range of artillery, into a "sea of fire".
Despite the harsh rhetoric, South Korea's Defence Ministry said there had been no suspicious behaviour by Pyongyang's army, adding that it was ready to defend the country against any attack.
The BBC's Nick Ravenscroft in Seoul says many analysts have predicted a new military provocation by the North, following last year's confrontation over a South Korean island and the sinking of one of its warships, for which the North denies responsibility.
These concerns were reinforced when military talks between the two Koreas broke down earlier this month, says our correspondent.
The US has about 30,000 troops stationed in South Korea. Technically North and South remain at war as the 1950-53 Korean War ended with an armistice, not a peace treaty.
The North has also been angered by a renewed propaganda drive from the South.
As well as activists sending leaflets criticising the North Korean leader Kim Jong-il via balloons floated over the border, it is understood the South's military has begun using the same technique - and may also be spreading news of uprisings in Arab countries.
Pyongyang calls this "psychological warfare" and has threatened to attack the sites from which balloons are launched.