North Korea has accused the US and South Korea of pushing the region to the "tipping point" of nuclear war.
Pyongyang was furious after the US deployed two bombers near its border in a show of force that it slammed as a "military provocation". It warned a nuclear war "would inevitably lead to another world war".
The US Air Force bombers dropped inert weapons over a training range in South Korea and fighter jets from both countries took part in rare live-fire drills.
Bombers reportedly flew close to the fortified border, which splits the Korean peninsula into two, and dropped 900kg bombs.
Tensions escalated in the peninsula after North Korea tested an intercontinental ballistic missile, believed to be capable of hitting targets as far away as Alaska.
Donald Trump's administration and North Korea's Kim Jong-Un have also exchanged hostile rhetoric for months.
North Korea, which is banned from testing or developing missiles under UN resolutions, has been working to develop a nuclear-tipped ICBM capable of hitting the US, ignoring repeated warnings from the international community.
The military drill by the US and South Korea saw American bombers destroy missile batteries and South Koreans jets deliver precision missile strikes on an underground command post, the AFP reported.
But an editorial titled "Don't play with fire on a powder keg" published in the state-run North Korean Rodong newspaper said the drill making war more likely.
Describing the Korean peninsula as "the world's biggest tinderbox", the article said: "The US, with its dangerous military provocation, is pushing the risk of a nuclear war on the peninsula to a tipping point."
The paper described the exercises as a "dangerous military gambit of warmongers who are trying to ignite the fuse of a nuclear war on the peninsula".
"A small misjudgment or error can immediately lead to the beginning of a nuclear war, which will inevitably lead to another world war," it added.
The article also justified North Korea's weapons' tests as "legitimate and justified measures" amid increasing "threats of nuclear war" against Pyongyang by Washington.
US military officials described the exercise as a defensive show of force and unity that demonstrated "the ironclad US commitment to our allies".
The US Missile Defence Agency said it would soon test an anti-ballistic missile system in Alaska.
A South Korean air force spokesperson also said the drill was a response to the series of ballistic missiles launched by North Korea.
Additional reporting by agencies.
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