North Korea's chief nuclear envoy warns negotiations on ending his country's nuclear programs are entering a crucial stage that could undo four years of diplomatic effort. As VOA's Heda Bayron reports from our Asia News Center in Hong Kong, his comments come ahead of the resumption of the six-nation nuclear talks.

North Korea's top nuclear envoy Kim Kye Gwan, right, shakes hands with unidentified people before leaving Pyongyang for Beijing, China

He told reporters at the Beijing airport that if North Korea, the United States, Japan, South Korea, China and Russia fail to agree on the details of the process, it could grind to a halt.

But Kim says if the United States and other countries meet their commitments, North Korea will do the same as well.

After four years of diplomatic effort, North Korea said earlier this month that it would dismantle its nuclear facilities by the end of the year. In February, the other five nations offered heavy fuel oil, economic aid and security incentives to North Korea, if it ended its nuclear programs. In July, it shut down its Yongbyon nuclear reactor.

But the details of how to move forward are still under negotiation. The U.S. and its allies want a verifiable and complete dismantling of the all the programs.

North Korea tested its first nuclear explosion a year ago.

The latest round of six-nation talks begins Thursday in Beijing.

On Tuesday, Kim also denied that North Korea has provided nuclear assistance to Syria.

He says "That matter is fabricated by lunatics. So you can ask those lunatics to explain it."

U.S. newspapers earlier this month reported that unnamed U.S. intelligence officials believe Syria and North Korea are cooperating on a nuclear facility in the Middle Eastern state.