North Korea has threatened to take pre-emptive action in response to US-South Korean military drills currently taking place in the region.
US and South Korean troops hold joint exercises every year
According to the official KCNA news agency, Pyongyang described the drills as "an undisguised military threat" and a "war action".
US and South Korean troops began the military exercises on Monday.
The drills are an annual event, and the North usually issues a strongly-worded statement against them.
But this year, tensions are higher than normal because of international anger at the North's recent decision to test-fire a series of missiles.
The US, which has had a military presence in South Korea since the end of World War II, currently has about 37,000 troops in the country, to complement the South's own force of 690,000.
North Korea has 1.2m troops, mostly stationed next to the heavily fortified border with the South.
The North Korean military "reserves the right to undertake a pre-emptive action for self-defence against the enemy, at a crucial time it deems necessary to defend itself", an army spokesman is quoted as saying by KCNA.
He added that the US-South Korean drills were a violation of the armistice that ended the 1950-1953 Korean War.
The two Koreas remain technically at war, because this armistice was never replaced by a formal peace treaty.
There is currently renewed international concern over North Korea.
It provoked an angry response when it test-fired seven missiles in July, and shows little sign of willingness to return to six-party talks on its nuclear ambitions.
In a sign of further tensions, US television network ABC reported recently that suspicious vehicle movements had been seen at a suspected North Korean test site, raising alarm about another possible test.