The U.S. envoy to six nation talks on disarming nuclear North Korea has said they reached agreement on the principles for verifying Pyongyang's denuclarization. However, the envoy, U.S. Assistant Secretary of State Christopher Hill, says negotiators still have a lot of work ahead of them. Daniel Schearf reports from Beijing.

After a long first day of resumed negotiations with North Korea, the U.S. envoy told journalists the six sides had an agreement on the mechanism for determining whether or not Pyongyang is hiding any nuclear facilities or materials, but only on principles.

He said they agreed that verification needs to include inspectors visiting nuclear sites, receiving documents, and interviewing North Korean nuclear staff. But, he said they agreed on few more details.

"The key thing will be tomorrow when the denuclearization working group meets to actually work out the actual protocol. So, they have a lot of work ahead of them," said Christopher Hill. "So, I hope they're getting a good night's sleep, better than we're getting here at midnight."

Hill said most of the six delegates, which also include China, South Korea, Japan, and Russia, held bilateral meetings before the heads of delegations met Thursday afternoon and into the evening.

He said on Friday the six parties would also discuss energy and aid to North Korea and he hoped they would also discuss the third and final phase of the agreed denuclearization that would see North Korea give up all its nuclear materials.

The six parties are still working through the second phase where North Korea is to declare all its nuclear materials and disable its nuclear facilities in return for aid and political incentives.

Pyongyang began destroying its main nuclear reactor and handed in its nuclear materials list last month, but there are suspicions the declaration is not accurate.

Officials in Washington have long held that North Korea has had a secret uranium enrichment program in addition to the plutonium program that was used to produce several nuclear weapons.

Hill said there was some discussion about the declaration and the heads of delegations gave opinions about the degree of its completeness, but he said he was not ready to discuss those details with the media.