Tens of thousands of South Korean and US troops took part in the Ulchi Freedom Guardian (UFG) joint military drills, a largely computer-simulated exercise that ran for two weeks in the South.
The annual drills are viewed by nuclear-armed Pyongyang as a highly provocative rehearsal for invasion, and it always meets them with threats of strong military counteraction.
The joint exercises began as North Korea and the US engaged in a war of words, which included President Donald Trump's apocalyptic warning to rain "fire and fury" on Pyongyang.
North Korea responded by threatening to launch a salvo of missiles towards the US Pacific territory of Guam -- a plan from which it later appeared to back off.
But on Tuesday, Pyongyang fired an intermediate range missile over northern Japan, with its state media calling it "a part of the muscle-flexing" against the war games.
Leader Kim Jong-Un, who oversaw the drills, warned the latest test was "a curtain-raiser" of the North's "resolute countermeasures" against the UFG exercises, the KCNA news agency said.
North Korea says it needs nuclear weapons to protect itself against the US and analysts say Pyongyang has made rapid strides in its ballistic technology in defiance of seven sets of United Nations resolutions.
In July it conducted two successful ICBM launches which appeared to bring most of the US mainland into range.
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