"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.

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."