WASHINGTON (AFP) - One of the top US experts on North Korea says President Donald Trump should agree to separating talks for a formal peace on the Korean peninsula from the issue of Pyongyang's denuclearisation.

In an interview with AFP, Dr Victor Cha, who was Mr Trump's pick for a new ambassador to Seoul last year before the White House changed its mind, said the President should get on board with the effort by North and South Korea to craft a declaration to end the 68-year official state of war between the two countries when their leaders meet in Pyongyang this week.

"The Chinese will probably support that," said Dr Cha.

"That puts Trump in a very awkward position, because there are three other parties that want a peace declaration, and he's the one who wants the credit, for the Nobel prize."

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

"The sequencing issue is not new," he said.

"They want a peace declaration and lifting sanctions first, we want steps towards denuclearsation first."

"We have to split up the negotiations."

Demand demilitarisation move by North

Since Mr Trump met Mr Kim in a groundbreaking summit in Singapore in early June, Washington has rolled together the two issues of denuclearisation 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 denuclearisation, says Dr Cha, now head of Korean issues at the Centre 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 towards a peace declaration, Dr Cha said, Mr Trump should get something in return for his endorsement.

Dr Cha says that could be a North Korean agreement to pull its artillery back from the heavily militarised border, 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," Dr Cha said.

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

"It's not real denuclearisation," said Dr 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."

The President retains some leverage, Dr Cha notes: Mr Trump's agreeing to the summit brought Mr Kim out on the world stage.

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

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