North Korea is still making nuclear material, six weeks after US President Donald Trump said the nuclear threat from Pyongyang was over.
US Secretary of State Mike Pompeo told the Senate Foreign Relations Committee that the country is still producing fissile material.
Mr Trump met Kim Jong-un in Singapore on 12 June, where the US said the North Korean leader had agreed to totally get rid of his nuclear weapons.
After the historic summit, Mr Trump tweeted that "there is no longer a Nuclear Threat from North Korea," but in recent days he has faced criticism about the lack of clear progress towards denuclearisation.
Mr Pompeo insisted that "progress is happening" and that Mr Trump is "upbeat about the prospects of North Korean denuclearisation."
The top US diplomat said Mr Trump "would agree that the primary systems that have threatened America continue to exist."
"I think what his comment was that the tension had been greatly reduced," Mr Pompeo said.
Still, he warned, the US will not let nuclear negotiations with North Korea continue indefinitely.
"We are engaged in patient diplomacy, but we will not let this drag out to no end," Mr Pompeo said.
He said he had emphasised this position during "productive" discussions he had had with Kim Yong-chol, Mr Kim's powerful right-hand man.
"Progress is happening. We need Chairman Kim Jong-un to follow through on his commitments made in Singapore."
In a sign of potential progress, new satellite imagery shows that North Korea has started dismantling a facility seen as a testing ground for intercontinental ballistic missiles.
Mr Pompeo added that "every single nation" must maintain enforcement of US sanctions against North Korea.
China and Russia have argued that North Korea should be rewarded with the prospect of eased sanctions for opening up dialogue with the United States and halting missile tests.
When asked if North Korea is still advancing its nuclear program, Mr Pompeo demurred.
"May I answer that question in a different setting?" he said, referring to a closed-door meeting with senators where classified information can be discussed.
