US President Donald Trump's special envoy to North Korea on Monday said it would be hasty to draw conclusions about satellite images that seem to show activity at a military testing facility Kim Jong-un pledged to dismantle.
Stephen Biegun was referring to pictures last week that analysts said appeared to show “deliberate and purposeful” rebuilding at the Sohae missile site.
Mr Kim last year promised to close the site when meeting South Korea's President, Moon Jae-in.
Mr Biegun led preparatory talks for Mr Trump and Mr Kim's second nuclear summit in Hanoi at the end of February. The talks failed to reach any agreement on ridding the Korean Peninsula of nuclear weapons.
But Mr Biegun insisted that diplomacy with Pyongyang was continuing, after what he said was a “fairly miserable record of achievement” in US efforts with the North for the past 25 years.
“We have a nuclear-armed state on the Korean Peninsula, so the policy has been a failure,” he said at the Carnegie Endowment for International Peace think tank in Washington.
“We have closed some gaps but we are not there yet. We are not going to do denuclearisation incrementally.
"The president has been clear on that. We are prepared to continue trying. The door is open.”
The summit in Vietnam fell apart early on the second day after North Korea asked for fast relief from sanctions before fully disarming.
Mr Trump said he chose to walk away because he was not willing to accept such conditions.
But the absence of any progress on an outline agreement from the first summit in Singapore last year led to criticism of the US president for lacking the preparation needed to make any deal.
Without progress on denuclearisation there are concerns that the process will stumble and break apart.
“Nothing can be agreed until everything is agreed," Mr Biegun said. "That is not to say that we cannot take steps to build confidence in the policy of denuclearisation.
“The short answer is, we don't know what Kim Jong-un will decide to do. We don't know that it is to send any particular message to us."
He was referring to the photos of the Sohae site released by the Centre for Strategic and International Studies in the US capital.
Researchers from the centre said their findings of renewed work at Sohae were obvious and not in dispute because the facilities there had been dormant since August last year.
“The tendency to reach these snap conclusions is a little hasty,” Mr Biegun said, adding that any rebuilding there by North Korea “would not be a productive step”.
But he admitted that the US still did not know the full extent of Mr Kim's nuclear programme, which is considered the communist leader's best strategic card.
His inner circle believes that retaining the nuclear programme regardless of any economic incentives offered by the US is essential to the regime’s survival.
“We need to agree on the definition of the North Korean nuclear programme. We need a full declaration,” Mr Biegun said.
“This is something we would like to get done in the president's first term. We have to get the North Koreans to start that process of denuclearisation. There is no artificial timetable."
