SEOUL (AFP) - North Korea test-fired four short-range missiles into the sea off its west coast on Friday in an operation supervised by leader Kim Jong Un amid heightened military tensions, a South Korean military spokesman said.

The missiles were launched from a base on the west coast and flew more than 100km, the spokesman for the South's joint chiefs of staff told AFP.

"The missile test launch, which lasted 45 minutes from 4:15 pm (0715 GMT), appeared to have been supervised by Kim Jong Un," he said, adding that the exact model of the projectiles was not known.

The exercise came at a time of elevated tensions with North Korea's condemnation of annual US-South Korea military drills that Pyongyang views as rehearsals for invasion.

One of the joint drills, Key Resolve, wound up last month, but the other, Foal Eagle, is set to continue until April 24.

The annual exercises 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 in early March by firing two short-range ballistic missiles into the sea off its east coast.

The nuclear-armed country also fired seven ground-to-air missiles into the sea off its east coast on March 13.

UN resolutions ban any ballistic missile test by North Korea.