Secretary of State Mike Pompeo said Sunday that North Korean leader Kim Jong Un’s vow to destroy his country’s nuclear-testing site is “a good first step” toward a summit agreement between Kim and President Trump, but repeated that both sides will have to take unprecedented steps.

“Kim understands this will have to be big and special,” Pompeo told “Fox News Sunday.” “I think Kim appreciates the fact this is going to have to be different. . . . If we can achieve an historic outcome, both sides have to come to play.”

However, he suggested that a phased-in or action-by-action type of negotiation between the leaders, as Kim has suggested, has historically failed.

“We’ve seen this before, and it’s failed,” Pompeo said.

Pompeo spoke after traveling last week to North Korea to negotiate with Kim on his final detail of the summit with Trump and to secure the release of three Americans imprisoned in the country.

Trump announced Thursday that the summit will be held June 12 in Singapore.

The president will trying to get Kim to give up his nuclear arsenal in exchange for economic support.

Pompeo also suggested Sunday, in his first interview since returning from North Korea, that the United States seeks a long-term agreement with Kim, not a so-called “regime change to replace him.

“Our hope is that Kim wants a strategic change, and President Trump is prepared to help,” the secretary said.

Pompeo also provided some specifics about how the U.S. might help North Korea, saying it would be through private-sector investment, not taxpayer money, and that it would focus on improving the country’s power grid and improve the Koreans’ food supply

Pompeo said last week that he told Kim while in Pyongyang that the United States aspires to have North Korea as a "close partner," not an enemy.

Pompeo, a Harvard Law graduate and former Kansas congressman, was Trump’s CIA director until the president nominated him in March to be secretary of state. The Senate last month confirmed Pompeo to the post as the country’s top diplomat.

On Tuesday, Trump said the United States was withdrawing from the international Iran nuclear deal, brokered in 2015 by the Obama administration. The U.S. will now re-impose sanctions on Iran, which remains in the deal with five other nations.

Pompeo has said talks last week with Kim were "warm," ''constructive" and "good" and that he made clear that if North Korea gets rid of its nuclear weapons in a permanent and verifiable way, the U.S. is willing to help the impoverished nation boost its economy and living stands to levels like those in prosperous South Korea.

"If North Korea takes bold action to quickly denuclearize, the United States is prepared to work with North Korea to achieve prosperity on the par with our South Korean friends," he said.

Pompeo repeated that position Sunday.

Since Trump announced plans to hold a summit with Kim, questions have been raised continually about whether the two leaders have the same objective in mind when they speak about "denuclearization." To the U.S., that means the North giving up the nuclear weapons it has already built. But North Korea has said it's willing to talk now because it's already succeeded in becoming a nuclear-armed state, fueling skepticism that the North would truly be willing to give up those weapons.

Pompeo said there would need to be "complete" and "verifiable" denuclearization that would remove North Korea as a threat to the rest of the world. He said a major inspection and monitoring regime would be required to ensure the North's compliance.

The Associated Press contributed to this report.