It's important that President Donald Trump "restrain himself" and build a relationship with Chinese President Xi Jinping because the U.S. needs China to help deal with North Korea, former United Nations Ambassador Bill Richardson told CNBC on Thursday.

A two-day summit between Trump and Xi was set to begin on Thursday in Florida, with North Korea expected to be a high priority.

While Trump has threatened to use trade to try to force China to exert influence over Pyongyang, he has also said he is willing to go it alone if China doesn't agree.

However, Richardson pointed out that North Korean leader Kim Jong Un is unpredictable, has nuclear weapons and ballistic missiles, and doesn't seem interested in having a dialogue.

"The only way, I believe, right now to get North Korea to the table is with China. They provide North Korea with food, energy assistance, all kinds of economic aid. They have the major leverage," Richardson said in an interview with "Closing Bell."

North Korea is working to develop an intercontinental ballistic missile capable of hitting the United States. This week, it fired a ballistic missile off its east coast.

Nicholas Burns, former U.S. Ambassador to NATO, told "Closing Bell" the most important thing right now is that Trump and Xi get to know each other.

"These are the two most important leaders of the world. Frankly, building some kind of relationship with Xi Jinping ought to be the first priority," Burns said.

However, he agrees with the Trump administration's stance to pressure China to use its leverage to convince North Korea to back down.

"That's a tall order. But I think the Trump administration is right to go there," Burns said.

However, even if China isn't on board, Burns thinks the U.S. shouldn't act unilaterally.

"I don't think the United States can or should go it alone. We have two very important allies in Japan and South Korea," he pointed out.

— Reuters contributed to this report.