The annual military exercises normally pass off without incident
North Korea has reacted angrily to the start of annual joint military exercises by the US and South Korea.
The American and South Koreans say the drills, which involve some 38,000 soldiers, are purely defensive.
But Pyongyang says the manoeuvres are preparation for an invasion and that its army is on full alert.
The war games come as international efforts continue to persuade the North to return to multi-party talks aimed at ending its nuclear programme.
Operations Key Resolve and Foal Eagle, which last 10 days, involve around 20,000 South Korea soldiers and some 18,000 from the US.
Such drills take place every year and usually pass off without incident, despite North Korean condemnation.
State media in the communist country warned that war "may break out at any time", and said military personnel had been ordered to "keep themselves fully ready to go into action in order to blow up the citadel of aggressors once the order is issued".
But Seoul dismissed the North's reaction as a "stereotype denouncement" and said that it had detected "no unusual military movement" north of the border.
"North Korea knows well that we mean no harm," said Defence Ministry spokesman Won Tae-jae.
North and South Korea remain technically at war after the 1950-53 Korean War ended without a peace treaty being signed.
North Korea pulled out of six-party talks on its nuclear programme and carried out its second nuclear test last year, after criticism of its long-range missile tests.
The US, China, Russia, Japan and South Korea speak regularly of their hopes that the North will rejoin the talks.