"I would do it at sometime, at sometime in a later future," Donald Trump said.

US President Donald Trump said Monday that it is "probably not" the right time for him to visit North Korea but he could see doing so sometime in the future.

"I don't think we're ready for that," Trump told reporters when asked about visiting Pyongyang for a meeting with North Korean leader Kim Jong Un. "I think we have a ways to go yet.

"The relationship is very good," Trump added, but the time was not right for a visit to Pyongyang.

"I would do it at sometime, at sometime in a later future," he said. "And depending on what happens, I'm sure he'll love coming to the United States also."

Kim asked Trump to visit Pyongyang in a letter sent last month, South Korea's JoongAng Ilbo newspaper reported Monday, repeating an invitation issued at their last meeting in the Demilitarized Zone (DMZ) dividing the two Koreas.

North Korea the same day demanded the United States provide security guarantees as a precondition to resuming deadlocked nuclear talks, dampening hopes for progress at a working-level dialogue expected in the coming weeks.

Nuclear negotiations between the two countries have been gridlocked since a second summit between Kim and Trump in February ended without a deal.

The pair agreed to restart working-level dialogue during an impromptu meeting at the DMZ in June, and Pyongyang last week offered to hold those talks in late September, a move the US said was "encouraging."

But hours later, Pyongyang carried out the latest in a series of weapons tests.