The top U.S. nuclear envoy on Tuesday called on North Korea to return to six-party talks aimed at dismantling Pyongyang's nuclear weapons program, but ruled out a one-on-one meeting with his North Korean counterpart.

"This is not about talks, this is really about action, and the action is they need to join the six-party process. We are ready. I've got a suitcase all set to go," U.S. Assistant Secretary of State Christopher Hill said ahead of a busy day of negotiations.

On his schedule were talks with counterparts from South Korea, China and Japan on the sidelines of a two-day security forum in Tokyo. Hill dismissed, however, the possibility of meeting North Korea's chief delegate, Kim Kye Gwan.

"I do not have any plans for more meetings," Hill said. "We can meet with DPRK any time, day or night, when they are in the six-party process." DPRK stands for the North's official name, the Democratic People's Republic of Korea.

Hill's comments came as delegates from all six nations involved in the talks _ the U.S., the two Koreas, China, Japan and Russia _ gathered in Tokyo for a private regional security conference.

Hill briefly encountered North Korea's Kim inside a building where the conference was held, but it wasn't a bilateral meeting, South Korean chief negotiator Chun Young-woo told reporters.

This week's security meetings have raised the possibility of restarting talks that have been stalled since last year on ending North Korea's nuclear program in exchange for aid.

Hill described his morning meeting with China's top nuclear envoy, Wu Dawei, as "very excellent" and said Beijing, the host of the six-nation nuclear negotiations, was committed to resolving the standoff through dialogue.

"We discussed the way forward in the six-party process," Hill said, without giving details. "We discussed some specific ideas about how we can make the process move ahead."

Hill met Chun earlier Tuesday and was holding talks with his Japanese counterpart, Kenichiro Sasae.

Chun said he was making efforts to help the United States and North Korea find common ground.

Chun told reporters after meeting with Hill that North Korea seems to be considering talks, but that "it is difficult to say at this point whether it will lead to a resumption."

North Korea's Kim later held a meeting with Sasae and said the two sides discussed "how to break the nuclear deadlock," but declined to give any details. The North Korean was scheduled to meet Wu later in the evening.

The forum's agenda will focus on energy, verification processes regarding the North's nuclear program and ways to ease tensions on the Korean Peninsula, according to the sponsors, University of California, San Diego.

North Korea's Kim said Monday he is prepared to meet bilaterally with the U.S., but has not backed away from his insistence that Pyongyang will return to the negotiating table only if the U.S. lifts financial sanctions.

North Korea has boycotted the six-party nuclear talks since November, citing what it calls a hostile U.S. attitude illustrated by the sanctions Washington imposed on North Korean companies for alleged financial crimes.

Washington maintains the sanctions are unrelated to the nuclear talks and will stay in place.