The allies held the rare live-fire drill as tensions grew over the peninsula following the North's first intercontinental ballistic missile (ICBM) test held last week.
The test sparked global alarm as it suggested North Korea now possessed an ICBM capable of reaching Alaska, a major milestone for the reclusive, nuclear-armed state.
Saturday's drill, designed to "sternly respond" to potential missile launches by the North, saw two US bombers destroy "enemy" missile batteries and South Korean jets mount precision strikes against underground command posts.
The North's state-run Rodong newspaper accused Washington and Seoul of ratcheting up tensions with the drill, in an editorial titled "Don't play with fire on a powder keg."
"The US, with its dangerous military provocation, is pushing the risk of a nuclear war on the peninsula to a tipping point," it said, describing the peninsula as the "world's biggest tinderbox."
During Saturday's drill, long-range B-1B Lancer bombers reportedly flew close to the heavily-fortified border between two Koreas and dropped 2,000-pound (900 kilogram) bombs.
Pyongyang described the joint drill 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 said.
Tension has been high as the US administration under President Donald Trump and the North's regime under leader Kim Jong-Un have exchanged hostile rhetoric for months.
Tension further escalated after Tuesday's ICBM test, a milestone in the North's decades-long quest for weapons capable of reaching the US.
The impoverished, isolated country has staged five nuclear tests -- including two last year -- and has made a significant progress in its missile capability under Kim, who took power in 2011.
In another drill held after the ICBM test, US and South Korean troops fired ballistic missiles simulating an attack on the North's leadership "as a strong message of warning," the South's military said at the time.
The US Missile Defense Agency said Friday it would soon test an anti-ballistic missile system in Alaska.
