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(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.

“There’s absolutely been no difference or distinction in the US policy on denuclearization,” he noted.

However, comments from the special representative prior to the summit seemed to suggest otherwise. Speaking in late January at Stanford University, Biegun denied that the US policy was “you do everything first and then we’ll begin to think about whether or not we’re going to do anything in response.”

“That is not our policy and has not been our policy. What we’re talking about is simultaneously looking at ways to improve relations, looking at ways to advance a more stable and peaceful, and ultimately, a more legal peace regime on the Korean Peninsula – how we advance denuclearization,” he said.

Last week, a senior State Department official said that “nobody in the administration advocates a step-by-step approach.”

Biegun, in multiple instances on Monday, failed to clarify the discrepancies in the pre- and post-summit negotiating stances.

“As is so often the case, nothing can be agreed until everything is agreed,” he said. “That’s a clear principle that has permeated our negotiations on both sides with the North Koreans.”

“That’s not to say that we can’t take steps to build confidence between the two countries, but the foundation of this policy is denuclearization. And until we can get to some point where we have the same traction on that issue that we have on the other issues, it makes it very difficult for us to move forward,” he continued.

Later when asked what those confidence building measures could be, Biegun confirmed that the US is looking to open a liaison office in North Korea in order to provide support to inspectors. Biegun noted that they “are not there yet” on that measure and said that it was “just one that’s been mentioned in public.” CNN reported in mid-February that this was under consideration.

The special representative also downplayed new satellite images that analysts say show activity at North Korean missile sites and urged against making any “snap judgment” on the significance of the images that appear to show that North Korea has begun rebuilding a portion of the Sohae facility previously used to test long-range missile engines.

“We don’t know” what Kim will decide to do in the future and that any decision to resume such testing may “very much be his decision, and his decision alone,” Biegun said. He also noted that Trump has “made clear” how disappointed he would be if testing resumed. A senior US defense official told CNN Monday that the commercial satellite imagery doesn’t show anything that raises imminent alarm for the US at this time.

CNN’s Jamie Crawford and Barbara Starr contributed to this report.