The South Korean military says it has held joint drills with the United States involving state-of-the-art stealth fighter jets.

Officials said aircraft from both countries, including stealth F-35A fighters, practiced intercepting cruise missiles and dealing with violations of airspace.

The exercises come at a time when North Korea is stepping up its confrontational stance. Leader Kim Jong Un recently described the South as his country's "invariable principal enemy."

Pyongyang is also test-firing cruise missiles at an unprecedented pace this year, apparently with the aim of mounting tactical nuclear warheads.

Experts say the North would use the weapons to attack the South Korean and US militaries, and Japan, should a contingency arise on the Korean Peninsula.