(CNN) The US special representative for North Korea said that the United States will not accept a phased denuclearization by Pyongyang and maintained that the two nations remain closely engaged despite the collapse of the Hanoi summit in late February.

"Let me start by saying the obvious -- that diplomacy is still very much alive," Stephen Biegun said at the Carnegie International Nuclear Policy Conference in Washington, DC on Monday. "While we haven't made as much progress in the six months as I would've hoped coming in on the first day, we stay closely engaged with our counterparts in North Korea."

Biegun, speaking less than two weeks after President Donald Trump and Kim Jong Un failed to reach any sort of agreement in Hanoi, suggested that the ball was in North Korea's court to negotiate in a meaningful way around the issue of denuclearization.

"We're just not there on denuclearization, and that was the issue at the summit that really challenged us to move forward with a more complete agreement," Biegun said.

Biegun said the administration's unanimous position was to not accept incremental denuclearization from the North Koreans and denied the US had hardened its position during negotiations in Hanoi.

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