"Right now, I think there is a lot of reasons for optimism that the negotiations will be fruitful, and we'll see," Mattis told Pentagon reporters as he greeted General Prawit Wongsuwan, the deputy leader of the Thai junta who also serves as defense minister.

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

The move comes 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.

Spokesman Colonel Rob Manning said the Pentagon has not changed its readiness to fight on the Korean Peninsula in light of recent developments.

"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," Manning said.

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.

Manning added that 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 earlier switched off giant loudspeakers blasting messages toward the North's soldiers at the border, in a conciliatory gesture ahead of Friday's talks.