KIM Jong-un has accused the US of preparing to invade North Korea as it kicks off its biggest war games in years on the border.
The dictator fired a flurry of missiles as he vowed to take "the toughest counteraction" against the joint drills with South Korea.
Kim ordered his troops to be on standby to respond to the "frantic war preparation moves" from the US and South Korea – which he views as an invasion rehearsal.
Although North Korea views the military exercises as a major security threat, America and South Korea said the drills are defensive.
The US has beefed up its alliances in the region amid North Korea's increasing nuclear threats, with more than 70 missile tests last year.
In the last year, Kim has been steadily expanding his nuclear arsenal and unleashed a barrage of banned missile tests.
The Warrior Shield FTX war games in South Korea include a computer simulation called the Freedom Shield 23, and field training exercises.
Pictures showed tanks and choppers readying for drills at a training field in the border city of Yeoncheon.
It's the biggest joint military exercise between the allies in five years.
Army Colonel Isaac L. Taylor, director of public affairs for US Forces Korea, said the war games will "build combat readiness and combined defence posture".
"This training is an additional example of our ironclad commitment to ensure security and stability on the Korean peninsula and across Northeast Asia," Taylor added.
As the drills kicked off, North Korea fired two cruise missiles from a submarine off its east coast in an apparent retaliation.
The move also signals the country will likely carry out more weapons tests during the 11 days of exercises.
The North's official Korean Central News Agency said the submarine salvo tested the country's "nuclear war deterrence".
It said the missiles flew more than two hours – drawing figure-eight-shaped patterns and demonstrating an ability to hit targets 930 miles away.
The missiles were reportedly fired from the 8.24 Yongung ship – the same sub that North Korea used to conduct its first sub-launched ballistic missile test in 2016.
Kim Dong-yub, a professor at the University of North Korean Studies in Seoul, said the chilling tests show how Japan – including US military bases in Okinawa – is within striking distance of the cruise missiles.
He said the weapons could reach even the US Pacific territory of Guam if a North Korean sub can operate further from its shore.
The tests were the North's first known launches of cruise missiles from a submarine as all its previous underwater launches involved ballistic missiles.
Moon Keun-sik, a submarine expert at Kyonggi University in South Korea, said: "At a time when its efforts to build (bigger submarines) have reported little progress due to the sanctions, North Korea wants to show its still almost developed the types of missiles that can be fired from a submarine."
Moon said the missiles were likely designed to strike approaching US warships or other shorter-range targets on the ground.
Experts said Kim is trying to pressure the US into accepting North Korea as a legitimate nuclear power and relax international economic sanctions.
Earlier this year, Pyongyang's foreign ministry warned the expansion of military drills threatened to turn the region into "a huge war arsenal and a more critical war zone".
It said the deployment of strategic assets was akin to planning a nuke strike on North Korea, and would "ignite an all-out showdown", KCNA reported.
It added the North was prepared to counter any challenge with the "most overwhelming nuclear force".
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