The U.S. envoy on North Korea acknowledged Wednesday that Pyongyang has not yet taken verifiable action to denuclearize and called on the regime to “seize the moment.”

Stephen Biegun, testifying to Congress on his nomination to be deputy secretary of state, also called for North Korea to step up its level of representation in talks.

“There is no meaningful or verifiable evidence that North Korea has yet made the choice to denuclearize,” Biegun told the Senate Foreign Relations Committee.

“We still believe that Pyongyang can make this choice,” he said. “The window is still open, but they need to seize the moment.”

President Donald Trump has met three times with North Korean leader Kim Jong Un, who has pledged to him to work to end his nuclear program.

While Trump has voiced admiration for Kim and said he believes he has his word, North Korea has carried out a series of rocket launches that have notably alarmed Japan.

Biegun last month met with a North Korean delegation in Sweden but said he has “yet to engage with an empowered negotiator across the table.”

He said that he would like to meet the North’s vice foreign minister, Choe Son Hui, saying she had Kim’s confidence and would be his counterpart.

Biegun said that Choe would be able to “sit across the table from me and make decisions about how we implement the vision that the two leaders agreed to in Singapore,” the first summit between Trump and Kim in June 2018.

A veteran Republican foreign policy expert who worked for years in Congress, Biegun appeared likely to be confirmed by the Senate.

Sen. Robert Menendez, the top Democrat on the committee, said that the nomination had “even greater significance” as Biegun stands to become “acting secretary of state for quite some time.”

Menendez was pointing to growing expectations that Secretary of State Mike Pompeo will resign in the coming months to run for Senate in his home state of Kansas.

Biegun was named to replace Deputy Secretary of State John Sullivan, who has been named ambassador to Russia.