North Korea says it will not allow samples to be taken from its nuclear facilities as part of international efforts to end its nuclear weapons programs.

A statement Wednesday from the North's Foreign Ministry says a request for samples would be considered to be the same as a house search and breach of North Korea's sovereignty.

The U.S. State Department, however, says that sample-taking is a key part of the verification process, adding that the North agreed to allow such checks at a meeting last month.

State Department spokesman Robert Wood says the North agreed to allow experts to take samples and remove them from the country for testing.

Agreeing on the verification process has been a persistent challenge for the six nations negotiating North Korea's nuclear disarmament, the United States, North Korea, South Korea, China, Russia and Japan.

Wood did not comment on the reasons for North Korea's apparent shift. But, a senior U.S. official admitted that the two sides are having issues with the verification process.

Last month, after the meeting was held and it appeared the two sides had made a breakthrough, U.S. officials said the six parties would meet soon in China to collectively adopt a U.S.-North Korea agreement on verification.

That has yet to happen.

North Korea has agreed to abandon its nuclear program in exchange for energy aid and other benefits. 

In his remarks to reporters Wednesday, Wood said that the U.S. was living up to its part of that agreement. Wood said that later this month and in early December, two shipments of heavy oil (50,000 tons) would arrive in North Korea.

He said the shipment was part of Washington's strategy of action for action. When it is delivered, Wood said the U.S. will have provided the North with 200,000 tons of heavy fuel oil.