North Korea's chief nuclear envoy met with U.S. experts late last week for informal talks in Mongolia over the resumption of stalled multilateral discussions on the North's nuclear program, a diplomatic source in Beijing said Monday.

The informal talks in the Mongolian capital of Ulaanbaatar on Friday were apparently aimed at reviving the deadlocked six-nation talks, but it was not immediately known whether any progress was made, the source said on the condition of anonymity.

The Friday talks involved Ri Yong-ho, Pyongyang's chief negotiator to the stalled six-party talks, and three U.S. experts, including Joel Wit and Robert Carlin, former U.S. State Department officials specializing in North Korean issues, the source said.

"Ri and the American experts met in Ulaanbaatar on May 23," the source said, declining to elaborate on what topics were covered.

North Korea and the U.S. held two rounds of such informal talks last year, which have served as venues for the two nations to share ideas about resuming the six-party talks.

Since North Korea's third nuclear test in February last year, the communist country has repeatedly expressed its willingness to reopen the six-party talks "without preconditions," but South Korea and the U.S. have maintained that North Korea must first demonstrate its sincerity toward denuclearization before the disarmament-for-aid talks can resume.

China has been more accommodating toward North Korea, urging South Korea and the U.S. to lower their bar for talks. Chinese Foreign Minister Wang Yi is set to visit South Korea on Monday with the agenda focused on how to deal with North Korea's nuclear program.

North Korea has been threatening to conduct its fourth nuclear test since last March, although recent satellite images showed no immediate signs of a test.

The six-party forum, which includes the two Koreas, the U.S., China, Russia and Japan, has been dormant since late 2008. (Yonhap)