North Korea has accused the US of preparing a pre-emptive strike on its nuclear facilities, as US-led forces continued to attack Iraq.
The US says it has no plans for military action against the North
The official Korean Central News Agency (KCNA) said large US-South Korean military exercises were clear signs that the US was preparing for war.
North Korea also warned South Korea to stop loudspeaker broadcasts across their heavily fortified border.
Earlier on Friday, in North Korea's first comment on the start of US-led military action in Iraq, KCNA said that newspapers had reported the start of the war, and predicted the conflict would have "disastrous consequences". It did not elaborate.
The US army has said drills on the Korean peninsula were planned nine months ago, but they come amid tension over Pyongyang's nuclear ambitions.
But KCNA said the US-South Korean annual military drills were "timed to coincide with the US attack on Iraq" and showed the US had a "win-win" strategy towards Iraq and North Korea - two countries which US President George Bush has named as being part of an "axis of evil".
North Korea also berated the South for allegedly resuming propaganda by loudspeaker at the DMZ. Both sides promised to halt slander attacks after the historic summit between both sides in 2000.
"If you do not take immediate measures to halt these damaging messages, then you will be completely responsible for the serious consequences that result," North Korea said to the South in a letter, KCNA reported.
South Korea's defence ministry confirmed receiving a letter but denied it was broadcasting propaganda across the demilitarised zone (DMZ).
"We usually broadcast music, the latest news on inter-Korean relations, the Iraq war and so on, but we don't broadcast anything slandering North Korea," a ministry spokesman said.
16 Oct: US says N Korea admits to a secret nuclear programme
14 Nov: US halts oil shipments to N Korea
22 Dec: N Korea removes monitoring devices at Yongbyon nuclear plant
31 Dec: UN nuclear inspectors forced to leave
10 Jan: N Korea pulls out of anti-nuclear treaty
12 Feb: IAEA refers issue to UN Security Council
27 Feb: US says Yongbyon reactor restarted
March 2: N Korean jets intercept US surveillance plane in international airspace
10 March: N Korea fires second missile into sea
North Korea and the US have been locked in an escalating standoff since October when the US said North Korea had admitted to a covert nuclear weapons programme.
The US cut off fuel aid to North Korea in protest.
Since then, North Korea has kicked weapons inspectors out of the country, reactivated a key nuclear facility and pulled out of the nuclear Non-Proliferation Treaty.
It has also tested two conventional short-range missiles.
North Korea has repeatedly called for direct talks with the US, but Washington has said it will only hold discussions with Pyongyang if the North's neighbours, Japan and South Korea, are involved.