Kim Jong Un hasn't met Trump yet 'so he better prepare for that': Former Trump adviser The June 12 summit will be the first time the U.S. and North Korea leaders meet.

A former adviser to President Donald Trump said North Korean leader Kim Jong Un hasn't yet met the U.S. leader and he “better prepare for” that.

Tom Bossert, an ABC News contributor and former White House adviser on homeland security and counterterrorism, told ABC News Chief Anchor George Stephanopoulos on "This Week" Sunday, that he has some advice for Kim ahead of the planned summit between him and President Trump.

“Just a thought here, we've all given advice to President Trump. I think a little advice to Kim Jong Un" is in order, Bossert said. “First, he's not met President Trump yet and he better prepare for that.”

Kim has met with a member of Trump's inner circle -- Mike Pompeo, who is now secretary of state and was formerly CIA director. But the planned June 12 summit would be the first time Kim and Trump meet.

Bossert's second piece of advice to Kim was about North Korea's recent, highly publicized destruction of a site where it had tested nuclear weapons.

“These demonstrations of blowing up tunnels, whether they're complete or whether they're facades, just for show, it's really not helping matters," Bossert said. "Even if they believe that to be a helpful gesture, it does make later confirmation of our [nuclear] inspectors, and what was in those tunnels all that more difficult. And so to build trust, I think that they should stop taking those unilateral actions at this point.”

Bossert appeared on "This Week" as part of a panel discussing the planned summit set for Singapore in just over a week.

Also on the panel was Bill Richardson, a former ambassador to the United Nations under President Clinton who has traveled to North Korea and negotiated with its government multiple times.

Richardson said that with the summit plans back on, “The danger now is that this will be a gigantic photo-op.”

“I'm glad the summit is reinstated,” he continued. “But you know, the North Koreans, I’ve negotiated with them. They maneuver you into a corner. They never say no. They’re relentless, they’re focused. And now our position has shifted dramatically. Now we are saying we are for and OK with a phased denuclearization.”

"We should have the summit,” Richardson added. President Trump “has to be dramatically prepared. Kim Jong Un will be very prepared. He knows his nuclear programs well.”

Richardson suggested that Trump should meet with Kim alone.

“I think the president, because he is a charming negotiator, he should take Kim Jong Un aside, because when you negotiate with the North Koreans, they never give any concessions across the table in a negotiating room," Richardson said. "Take him out informally. Find a way to build some trust with the guy. You know he can't be trusted... But find some time [for a] one-on-one, together, to build trust, to find a way to have a phased denuclearization, but one that has timelines, disclosure of their arsenal.”

He added, “The president should avoid hugs, smiles, photo-ops as much as he can. That is going to be difficult.”

Another member of the panel, Sue Mi Terry, a senior fellow for the Center for Strategic and International Studies and a former CIA analyst on Korea issues, said it isn't "realistic" to expect North Korea to accomplish complete, verifiable and irreversible denuclearization.

“We still have to set it up as a goal,” Terry said to Stephanopoulos. But, she added, "We don't know where all the weapons are, how many they have, where they're all hidden.”

“There are thousands of underground tunnels where they are hidden, so [any] verification process is going to be very, very difficult to achieve. So we need to have a realistic expectation," Terry said.

Trump announced Friday that his planned meeting with Kim Jong Un in Singapore is back on after the president had cancelled it a little more than a week before.

The president also hinted at the possibility for more U.S.-North Korea meetings in the future, saying, “I think it’ll be a process. I never said it was going to be just one meeting.”