"We are not going to do denuclearization incrementally," said Stephen Biegun, the special US envoy for North Korea.

While stressing he was not bound by an artificial calendar, Biegun said the administration wanted to accomplish the complete and verified denuclearization of North Korea by the end of Trump's term in January 2021.

"We stand by the expectation that if we fully mobilize our resources... we could align ourselves in a manner sufficient to achieve this in something approaching a year," he said.

"We do have to have a complete declaration," from Pyongyang, he went on. "We will not lift these sanctions until NK completes the process of denuclearization."

The US side has included North Korea's chemical and biological weapons in the negotiations with Pyongyang.

The envoy said there could be no comprehensive agreement with the North without agreement on each point, adding that there was "complete unity" in the administration on that approach.

Speaking at a conference organized by the Carnegie Endowment for International Peace, Biegun said progress had been made despite the failure of Trump's second summit with North Korean leader Kim Jong Un, held in Hanoi last month.

"The door remains open" for further negotiations, he said, adding, however, that "the North Koreans have to be fully invested."

"Nothing can be agreed until everything's agreed," he stressed, while adding that confidence-building measures between the countries could still take place.

The outcome in Hanoi last month fell far short of hopes that the meeting would build on the leaders' summit in Singapore in June 2018, which marked a historic first but resulted only in a vague commitment to "work toward complete denuclearization of the Korean peninsula."

After weeks of building expectations and with a signing ceremony ready to go, Trump abruptly ended the summit, saying: "Sometimes you have to walk and this was just one of those times."

North Korean Foreign Minister Ri Yong Ho later said his side had offered to "permanently and completely dismantle all the nuclear production facilities" at its main complex in Yongbyon if the US dropped sanctions "that hamper the civilian economy and the livelihood of our people."

Biegun repeated the US position that sanctions would not be lifted "until North Korea completes the process of denuclearization."