"Discussions are ongoing and they're going very, very well," Trump told reporters.

"We have no time limit. We have no speed limit."

Trump said he discussed North Korea with Russian President Vladimir Putin on Monday at their summit in Helsinki.

"President Putin is going to be involved in the sense that he is with us," Trump said.

The Republican president met with Kim on June 12 for an unprecedented summit in Singapore during which the North Korean leader repeated a pledge to denuclearize his country.

But the accord did not spell out a timetable for the process or say how it would be carried out. Diplomats are now expected to hammer out the details.

More than a month later, no concrete progress has been reported and North Korea has complained the Americans are making unilateral demands.

Before the Singapore summit, the Trump administration said denuclearization should start "without delay," and after the meeting, it spoke of the process beginning "very quickly."

A day after the meeting, US Secretary of State Mike Pompeo said the bulk of North Korea's denuclearization should be completed by the end of Trump's term in 2020.