North Korea would breach an informal understanding with the United States by restarting its nuclear weapons testing program, acting White House chief of staff Mick Mulvaney said Sunday.

Mulvaney was on "Fox News Sunday" and told host Chris Wallace the Trump administration would be disappointed if the North Korean regime under leader Kim Jong Un started testing missiles again.

"I think the resumption of the missile testing would be seen as sort of a violation of some sort of breach of trust," Mulvaney said. "I think there was a general understanding that there was no reason for that to continue, as long as we were continuing to have conversations. And the conversations continue."

Kim and President Donald Trump recently met in Hanoi, Vietnam, their second face-to-face meeting, but their talks to put an end to North Korea's nuclear weapons program ended without an agreement.

Mulvaney said hashing out an agreement that satisfies both sides is not easy and will take more time.

"We did not get a deal in Vietnam. I think those folks who thought it might be easy for us to get a deal don't understand the complexities of the issue. It took Reagan and [Mikhail] Gorbachev many, many times to solve just a piece of the nuclear weapons problem. To think we will solve the Korean issue in one meeting or two meetings is probably not reasonable," he said.

"The discussions can and should continue, I can foresee the president and the chairman sitting down at some point in the future. But if they were to begin testing again, that would be seen as a truly disappointing turn of events."

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