The chief U.S. negotiator on North Korea said Saturday in Tokyo that he expects Pyongyang to shut down its Yongbyon nuclear facility "probably in three weeks."

Assistant U.S. Secretary of State Christopher Hill said he thinks the plutonium-generating reactor will be boarded up after North Korea and the International Atomic Energy Agency reach an agreement on how to monitor the shutdown process.

"We expect this to take place soon, within probably in three weeks," said Hill, who was in Tokyo to brief his Japanese counterpart, Kenichiro Sasae, on his surprise trip to North Korea on Thursday and Friday to push Pyongyang to abandon its nuclear weapons program.

His Pyongyang trip came on the heels of recent positive developments in the nuclear standoff, including a breakthrough on the transfer of disputed North Korea-linked funds and North Korea's subsequent invitation for IAEA inspectors to come in.

He said he had "good discussions" with North Korean leaders, adding he urged them to improve relations with Japan, the world's second largest economy, by resolving issues related to the abductions of Japanese nationals. However, there were no new response from the North Korean side, he said.

Hill also said the next six-party meeting on the North's nuclear program should be held only after Pyongyang starts the shutdown process at Yongbyon, the first step North Korea agreed to take under a February agreement on denuclearizing the reclusive state.

"We want the six parties to discuss the next steps," Hill said.

U.S., Japan, China, Russia and South Korea have jointly pressured the North to abandon its nuclear program in the six-party talks.