In a highly unusual late-night statement at the North Korean delegation's hotel in Hanoi, foreign minister Ri Yong Ho said Pyongyang had made a "realistic proposal" at the summit.

He denied Trump's assertion that Pyongyang had demanded the lifting of all sanctions imposed on it over its nuclear weapon and ballistic missile programmes.

If the US lifted clauses that "hamper the civilian economy and the livelihood of our people" from five UN Security Council resolutions, Ri told reporters, "we will permanently and completely dismantle all the nuclear production facilities in the Yongbyon area, in the presence of US experts".

Pyongyang holds the US responsible for the UN Security Council sanctions imposed on it over its banned weapons programmes.

"This proposal was the biggest denuclearisation measure we could take at the present stage in relation to the current level of confidence between the DPRK and the United States," he added, using the initials of the North's official name.

The North's stance was "invariable" and its offer would "never change", even if the US proposed more negotiations in future, he said.

The minister was reading a statement after the two-day Trump-Kim talks closed with no final deal and the US president gave a press conference before flying out of Vietnam.

The much-anticipated second summit between the two leaders ended with no agreement reached and a scheduled signing ceremony was dropped.