Doing so would mean Trump backing off his demand that North Korean leader Kim Jong Un first take concrete steps toward giving up his nuclear weapons. But Trump should insist on something concrete in return, Cha said.

"The sequencing issue is not new," he said. "They want a peace declaration and lifting sanctions first, we want steps towards denuclearization first."

"We have to split up the negotiations."

Since Trump met with Kim in a groundbreaking summit in Singapore in early June, Washington has rolled together the two issues of denuclearization and an official end to the hostilities that began with the 1950-53 Korean War.

Since then there has been no sign of Pyongyang truly moving on denuclearization, says Cha, now head of Korean issues at the Center for Strategic and International Studies in Washington.

"We want a declaration (of nuclear facilities), we want verification, we want a timeline.... There is nothing that I've seen that shows North Korea wants to do any of those things."

If North and South Korea do move toward a peace declaration, Cha said, Trump should get something in return for his endorsement.

Cha says that could be a North Korean agreement to pull its artillery back from the heavily militarized border, from where it can easily strike densely populated Seoul.

"If we're going to do a peace declaration, we have to get something, something that's valuable," Cha said.

Trump "might be very tempted to do it, to follow them and then to take control of it, to say 'it was all my idea, this is all going really well,'" he said.

As for denuclearization, Cha is less optimistic that a strong deal can be achieved. Pyongyang wants the US to lift economic sanctions first, and so far is only willing to take modest measures like closing testing sites.

"It's not real denuclearization," said Cha.

He noted recent US intelligence reports that indicate Pyongyang is actually now making more fissile material and more weapons.

"The real question is can we get a good deal, one pretty comprehensive and that is verifiable? That's a much harder question to answer, because I don't think the North Koreans are interested in giving up their weapons."

Trump retains some leverage, he notes: Trump's agreeing to the summit brought Kim out on the world stage.

"Before he was an isolated leader, he was ignored, nobody cared about him," said Cha.

"Is he willing to make a deal because he doesn't wanna go back to being isolated? We don't know." (AFP)