In the latest dispute to hit a six-nation disarmament deal, North Korea said Wednesday it had never agreed to let inspectors take samples from its atomic plants.

The US State Department insisted that the North did consent to sampling.

The dispute over ways to verify the North's declared nuclear programme is just the latest hurdle in tortuous negotiations, which began in 2003 and have often come close to breakdown.

South Korea's Foreign Minister Yu Myung-Hwan said Seoul had pointed out ambiguities surrounding verification methods in an agreement reached in early October between Washington and Pyongyang.

"Washington itself admitted a lot of ambiguities exist," Yu told parliament.

"We'll take our time to gauge the North's intentions. Through consultations with other countries, we will ensure that sampling is an essential part of verification," he said.

Yu stressed that the North did not intend to leave the six-party process.

Earlier in the day Yu described the North's sampling ban as disappointing.

"The sampling issue is the core focus of the verification measure," he told local editors. "The US and North Korea held their recent talks on this understanding."

Yu said South Korea's nuclear envoy Kim Sook and his US counterpart Christopher Hill held telephone talks earlier Thursday on Pyongyang's stance.

"The US will have additional contacts with North Korea to ascertain its intentions," he said.

The six-nation forum -- the two Koreas, the United States, China, Russia and Japan -- reached agreement on a broad denuclearisation pact last year.

Under the agreement, the North, which tested an atomic weapon in 2006, was to disable the plants at Yongbyon that produced weapons-grade plutonium and declare all its nuclear activities. In return it would receive energy aid and diplomatic benefits.

The pact nearly collapsed this autumn over verification procedures until Hill travelled to Pyongyang in early October to negotiate a compromise.

US officials said at the time that outside experts would be allowed to visit both declared and undeclared sites, take and remove samples and equipment for analysis, view documents and interview staff.

However, visits to sites not included in the North's declaration handed over in June would require "mutual consent."

The North said Wednesday that verification would be confined to Yongbyon and would involve only field visits, confirmation of documents and interviews with technicians.

The State Department insisted later Wednesday there had been "understandings" that experts could take samples out of the country for testing.

US officials say sampling is crucial to checking how much bomb-making plutonium the North produced in the past -- and how many bombs it could theoretically make.

North Korea was promised one million tons of fuel oil, or energy aid of equivalent value, in return for disabling its plants but only about half of this has been received.

The North said Wednesday said it had slowed down disablement work in protest.

"In case the economic compensation continues to be delayed, the tempo of the disablement will be decreased accordingly, making it hard to predict the prospect of the six-party talks," it said.

The State Department said it has arranged to ship another 50,000 tons of oil by next month.

All rights reserved. © 2005 Agence France-Presse. Sections of the information displayed on this page (dispatches, photographs, logos) are protected by intellectual property rights owned by Agence France-Presse. As a consequence, you may not copy, reproduce, modify, transmit, publish, display or in any way commercially exploit any of the content of this section without the prior written consent of Agence France-Presse.