After U.S. nuclear envoy Christopher Hill's visit to Pyongyang in early October, Washington said the sides had reached a verbal agreement on verifying North Korea's nuclear activities, under which Pyongyang pledged to allow foreign experts to take samples from the Yongbyon complex.
"It is an act of infringing upon sovereignty, little short of seeking a house-search ... to insist on adding even a word to the written agreement," the Foreign Ministry said in a statement released by the official Korean Central News Agency.
North Korea's refusal to allow sampling, which the U.S. considers to be a key step in verifying the secretive state's past nuclear activities, is likely to put a new obstacle in the way of six-party negotiations on the North's denuclearization, involving the two Koreas, Japan, China, the United States and Russia.
The statement said North Korea agreed to foreign inspectors visiting the Yongbyong complex, conducting tests, examining documents and interviewing scientists, but only after the five countries fulfill their commitments under a six-party deal to provide fuel aid.
The ministry also said work to deconstruct the reactor has been slowed down.
"We have taken measures to slow down to half the speed of removing spent fuel rods according to the action-to-action principle, in response to the delay in economic compensation by the five parties," the statement said.
The North Korean government has issued several angry statements in recent weeks against South Korea, for its alleged failure to honor intergovernmental commitments and to prevent the distribution of propaganda leaflets, and against Japan, for its refusal to provide promised fuel aid and over Japanese media rumors on the health of North Korean leader Kim Jong-il.
The North Korean government said earlier on Wednesday that it would close its land border with the South as of December 1, due to Seoul's "grave and wanton violations" of the 2000 and 2007 inter-Korean summit agreements.