Pyongyang branded the drill a “dangerous military provocation” in a region it dubbed the “world’s biggest tinderbox”.
The show of force comes amid growing tensions on the peninsula after the nuclear-armed North launched its first intercontinental ballistic missile that some experts believe could be capable of reaching Alaska, and parts of the US West Coast.
The US exercise, designed to “sternly respond” to potential missile launches by the North, saw the bombers drop the inactive weapons on a training range in South Korea.
South Korean and American fighter jets also took part in the drill.
Japanese fighter planes flew with the bombers on their way back to Andersen Air Force Base in Guam.
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”.
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 misjudgement or error can immediately lead to the beginning of a nuclear war, which will inevitably lead to another world war,” it said.
US military officials described the exercise as a defensive show of force that demonstrated “the ironclad US commitment to our allies”.
It comes as President Donald Trump pressed for action at the recent G20 summit to tackle “the problem and menace of North Korea”.
Japanese Prime Minister Shinzo Abe also said the security situation in the Asia Pacific region had become “increasingly severe”.
The isolated authoritarian state has staged five nuclear tests, including two last year, and has been making a concerted effort to develop its missile capability.
The recent ICBM test marked a milestone in its weapons programme and caused global alarm.
In response to the launch, US and South Korean troops fired ballistic missiles in a simulated attack on the North’s leadership, in what was described “as a strong message of warning”.
The US Missile Defense Agency has said it would shortly test an anti-ballistic missile system in Alaska.