The US has about 28,500 troops based in South Korea, as well as a large military presence constantly monitoring the region's skies and oceans.

"From a military perspective from the Department of Defense, there has been no change in our posture and our determination to be ready to fight tonight," Pentagon spokesman Colonel Rob Manning said.

North Korean leader Kim Jong Un on Saturday declared Pyongyang had no further need for nuclear tests or intercontinental ballistic missile launches, and no further use for its atomic test site.

Manning said that despite the move, the US has not changed its conditions "with the demand for the complete, irreversible and verifiable denuclearization of the Korean Peninsula."

South Korea's military on Monday switched off giant loudspeakers blasting messages towards the North's soldiers at the border, in a conciliatory gesture ahead of Friday's historic inter-Korea summit between Kim and South Korea's President Moon Jae-In.

Kim is expected to meet US President Donald Trump in late May or early June.