North Korea can be trusted to denuclearize only if its promises can be verified and China participates in the treaty, Colorado U.S. Sen. Cory Gardner said Friday.

Gardner, a Republican who chairs the Senate Foreign Relations subcommittee on East Asia, the Pacific and International Cybersecurity Policy, has long advocated a hardline toward North Korea.

On Thursday, President Donald Trump agreed to accept a surprising invitation from North Korean leader Kim Jong-un for a personal meeting to discuss the possibility the staunch Communist country would give up its nuclear weapons program. U.S. intelligence sources estimate North Korea has about 120 nuclear warheads, some of which could reach the United States in missiles.

Gardner’s expertise on North Korea has made him a frequent source for television, radio and newspaper interviews since the president accepted North Korea’s invitation. The date for the meeting has not yet been set.

Gardner said during an interview on MSNBC’s Morning Joe that “our number one goal has to be the complete, irreversible, verifiable denuclearization. … I spoke to the president last night and it sounds like that is exactly what he intends to do.”

He added, “If the result of this meeting is not verifiable concrete steps to denuclearization, then it will be a failure, and then the president is going to be under tremendous pressure because that new red line has been set to act.”

Trump’s “maximum pressure doctrine” that has included threats of war likely led to Kim’s decision to negotiate, Gardner said. He added that Trump’s tough talk was an improvement compared with President Barack Obama’s “failed strategic patience doctrine.”

He also said the negotiations would succeed only if China participates “in a way that they have never done before.”

China is North Korea’s main international trading partner and one of its few allies.

In a different interview with National Public Radio Friday, Gardner seemed skeptical of  North Korea’s willingness to abide by international agreements to denuclearize.

“In 1994 they made agreements that broke off in the early 2000s,” Gardner said. “In 2005 and 2007, they made agreements that broke off.”

Gardner and other Republican senators wrote a letter to the president Thursday urging caution with North Korea. It said he should continue sanctions and joint military exercises with South Korea until dismantling of North Korea’s nuclear weapons program could be proven.

“While we must take any credible opportunity to talk with Pyongyang about denuclearization, we must also never forget that the [Democratic People’s Republic of Korea] continues to represent a grave threat to the United States, our allies, and global peace and stability,” the letter said.

Last October, Gardner co-sponsored a Senate bill to impose sanctions on North Korea because of its nuclear weapons program and human rights abuses. Among its provisions, it would block financial institutions and other organizations that deal with North Korea from conducting business in the United States.