"We will adjust our exercise posture, either more or less, depending on what diplomacy may require," Esper said, adding, "We have to be open to all those things that empower and enable our diplomats" in the nuclear talks.

North Korea has long objected strongly to large-scale American and South Korean military exercises, which it calls preparations for an invasion of the North. President Donald Trump also has criticized the exercises as too costly and provocative, but US military commanders consider them crucial to deterring North Korea and ensuring that any invasion by the North would fail.

"As we consider adjusting -- either dialing up or dialing down -- exercises, training, stuff like that, we want to do that in close collaboration with our (South) Korean partners, not as a concession to North Korea but, again, as a means to keep the door open to diplomacy," he said.

Esper said he takes seriously North Korea's statement that the end of this year is a deadline for the US to change its approach to the nuclear negotiations.

He said he is hopeful that diplomacy will prevail, given the history of tensions on the Korean Peninsula since the North Koreans began launching intercontinental-range ballistic missiles that could eventually be nuclear-armed.

Esper recalled his concern about the prospects for war on the peninsula when he became Army secretary in 2017.

"We were on the path to war," he said. "It was very clear to me because the Army was making preparations." He did not elaborate.

The US has about 28,000 troops in South Korea, and Esper said they must be ready to fight the North at a moment's notice.

Esper also said that during his talks in Seoul this week with his South Korean counterpart, he will express US concern about Seoul's stated plan to withdraw from an intelligence-sharing pact with Japan. Esper said the dispute between Tokyo and Seoul is only helping North Korea and China. (AP)