South Korea and the United States wrapped up Friday one of their two large-scale annual military exercises that have prompted bellicose warnings and threats from an incensed North Korea.

The Key Resolve exercise, which began on March 2, involved around 10,000 South Korean and 8,600 US troops, but is a largely computer-simulated command post drill that rehearses various conflict scenarios.

General Curtis Scaparrotti, commander of the allies' Combined Forces Command, said Friday the drill was "critical" to ensuring the defensive security of South Korea.

North Korea views Key Resolve and the eight-week Foal Eagle joint exercise that kicked off at the same time, as provocative rehearsals for invasion.

The annual drills always trigger a surge in military tensions between the two Koreas who remain technically at war because the 1950-53 Korean conflict ended with a ceasefire rather than a peace treaty.

North Korea signalled its displeasure when this year's drills began by firing two short-range ballistic missiles into the sea off its east coast.

UN resolutions ban any ballistic missile test by North Korea, and Seoul's defence ministry warned that Pyongyang appeared intent on triggering a "security crisis".

Although the North has kept up its rhetorical condemnation of the drills, there have been no further missile launches or other incidents.

There were some concerns that Pyongyang might consider conducting a fourth nuclear test, but analysis of recent satellite images of the North's main tests site shows no signs of preparations for an imminent detonation.