Washington/Seoul  - The United States on Wednesday criticized North Korea's refusal to allow samples to be taken to verify its nuclear disarmament as the country again slowed the dismantlement of its nuclear facilities.

US State Department spokesman Robert Wood told reporters in Washington that in a deal made last month, the United States and North Korea had "basically agreed that experts could take samples and remove them from the country for testing."

He also added that the United States kept its part of the deal by shipping 500,00 metric tons of heavy fuel oil to the impoverished communist state.

North Korea's Foreign Ministry denied Wednesday that allowing sampling was part of the October agreement on verification procedures, saying a US demand for environmental and waste sample taking was a breach of sovereignty that would "certainly bring about a war."

South Korean Foreign Minister Yu Myung-hwan said Thursday that the top nuclear negotiators of South Korea and the United States were discussing the issue.

"The sampling issue is the core focus of the verification measure," Yu told South Korean media.

Yu said Pyongyang might hope to press further concessions from the United States and the international community by barring international inspectors from doing their work at its Yongbyon nuclear facility, which produced weapons-grade plutonium.

Pyongyang's official Korean Central News Agency said late Wednesday that the country slowed down disassembling its nuclear facility at Yongbyon because promised economic aid did not arrive on time.

Work to remove spent fuel rods was slowed down by half in an "action-for-action strategy," a Foreign Ministry spokesman was quoted as saying. Should there be further delays, dismantlement work would slow accordingly, he said.

The United States stressed that it had kept its end of the deal with the heavy oil shipments set to arrive in North Korea in late November and early December.

"The United States is doing its part with regard to action-for-action, and we're going to continue to have these discussions with the North on verification," Wood said.

North Korea last month resumed dismantling its nuclear facilities, having halted work until it was removed from a US terrorism blacklist.

The country agreed to give up its nuclear weapons programme in a deal cut with the United States, South Korea, China, Japan and Russia in exchange for economic and energy aid.

In another move set to further chill the already frosty bilateral relationships with South Korea, the North announced plans Wednesday to close its land border with its neighbour beginning December 1 in retaliation for South Korean activists dispatching balloons filled with propaganda leaflets across the border. (dpa)