The chief U.S. nuclear envoy expressed optimism after a day of meetings Tuesday with his North Korean counterpart, saying no new obstacles to progress on disarmament have emerged following Pyongyang's shutdown of its sole operating reactor.

"I think we're all in the same ballpark," Assistant Secretary of State Christopher Hill said after the meetings with Kim Kye Gwan ahead of nuclear talks in Beijing.

"We had a good discussion -- at this point there are no show-stoppers," he told reporters.

Hill repeated his desire to have the North disable its nuclear facilities by the end of the year, following its weekend shutdown.

"I laid out my view on how this could be done, and I think we had a good discussion on that basis," he said.

Hill said he would meet Wednesday ahead of the formal resumption of six-nation negotiations with the Chinese hosts to gauge whether the proposed timeframe was feasible.

"I think people are feeling pretty confident about our six-party process," he said.

The shutdown of the Soviet-era Yongbyon reactor that produces plutonium for use in bombs was North Korea's first step toward halting production of atomic weapons since the standoff began in 2002. The North conducted its first nuclear test explosion in October.

Talks this week were expected to focus on setting a schedule for North Korea to declare all its nuclear programs and then disable them so they cannot be easily restarted. The North has pledged in principle to eventually abandon its nuclear facilities.

Before leaving Pyongyang on Tuesday, Kim told APTN that closing the reactor meant the process was moving into a second phase.

"There should be discussion on how to define the targets of the second phase, the obligations for each party, and also the sequence of the actions," he said.

South Korean nuclear envoy Chun Yung-woo said the closing of the nuclear reactor was important, but only a first step.

"There is a very difficult and steep road ahead of us. We need to make sure that North Korea won't become hesitant or lose interest in going up that difficult and steep road," he said after arriving in Beijing.

"We need to discuss thoroughly how to push the process forward," Japanese envoy Kenichiro Sasae said.

Inspectors from the International Atomic Energy Agency have verified the reactor closure and were checking other facilities -- including two dormant construction sites for reactors -- and installing monitoring equipment.

North Korea has begun receiving 50,000 tons of oil from South Korea as a reward for the shutdown, and is to eventually receive the equivalent of 1 million tons for disabling the facilities, which the U.S. hopes is completed this year.

The timing of the detailed steps to accomplish that have yet to be decided. The sides also have not yet agreed on what to do with North Korea's existing plutonium stockpile, believed to be enough for about a dozen bombs.

Looming as a first obstacle will be North Korea's declaration of its nuclear facilities and whether it publicly acknowledges the uranium enrichment program that the U.S. accused it of having in 2002, sparking the nuclear crisis.