North Korean leader Kim Jong Un "better prepare" for the meeting with President Donald Trump, former Trump adviser Tom Bossert said on ABC's "This Week" on Sunday.

"We've all given advice to President Trump, said Bossert, a former White House adviser on homeland security and counterterrorism. "I think a little advice to Kim Jong Un in this case. He has not met President Trump yet and he better prepare for that."

He went on to explain that Trump has been moving quickly on the North Korean issue, because he is "trying to ascertain the motive and intent of the leadership. If they don't have the intent to make a strategic decision about that means to disarm as a first step, not freeze but halt and disarm... then this is a big waste of time. All the definitions and all the big protracted negotiations that might come after are all for naught."

Bossert's comments came after Bill Richardson, a former ambassador to the United Nations under President Bill Clinton who has negotiated with the North Korean government numerous times, told the "This Week" panel discussing the planned summit that Trump "has to be dramatically prepared. Kim Jong Un will be very prepared. He knows his nuclear programs well."

Richardson emphasized that "The danger now is that this will be a gigantic photo-op."

He said it is good that the summit will take place but warned that the North Koreans "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."

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, cautioned that Trump "cannot wing it. He needs to be disciplined. North Korea spent all their energy, 100 percent of their focus on the United States and Kim Young Chol, [the North Korean official who just visited the White House] was part of the negotiating process since 1992. These guys are prepared. They know what they are talking about and they know what they want."

She also stressed that "We still do not have a definition of exactly what denuclearization is… is North Korea willing to denuclearize as we define it?"

She also said that the North Koreans have "thousands of underground tunnels where [weapons] are hidden, so [any] verification process is going to be very, very difficult to achieve. So, we need to have a realistic expectation."

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