US military exercises with South Korea could be scaled back to aid diplomacy with the nuclear-armed North, US Defense Secretary Mark Esper said on his way to Seoul, reports AFP.

"We will adjust our exercise posture either more or less depending on what diplomacy may require," Esper told reporters on board his plane to Seoul. The possible downsizing of the joint drills should not be seen as a "concession" to Pyongyang, he said, "but as a means to keep the door open to diplomacy".

His comments came after Pyongyang reiterated its demands for the combined exercise to be scrapped.

Negotiations have been gridlocked since the Hanoi summit between Kim and Trump broke up in February with disagreement over sanctions relief and what the North would be willing to give up in return.