The latest deadlock over North Korea's nuclear disarmament will be hard to break because it involves a key test of how much bomb-making plutonium has been produced, a South Korean official said Thursday.

The senior official gave details of the dispute which led the North to announce on Tuesday that it has stopped work to disable its nuclear plants.

The US and the North cannot agree on procedures to verify the disclosure which the North made in June of its nuclear activities as part of a six-nation deal.

The US refuses to remove the communist state from a terrorism blacklist until there is an agreement.

The Seoul official, speaking to AFP on condition of anonymity, said the "bone of contention" is the US demand to conduct sampling of nuclear material in the North.

While the United States insists that outside inspectors be allowed freely to collect samples from nuclear facilities or equipment, North Korea rejects the demand, the official said.

He said Pyongyang would allow outside inspectors merely to tour facilities, interview engineers and scientists and look at relevant documents -- which Washington says is not good enough.

The US says the sampling work is crucial to verifying the amount of plutonium which the North produced in the past.

The North tested an atomic weapon in October 2006 before reaching the aid-for-disarmament deal last year.

Its June declaration has not been made public but some media reports say the North puts its plutonium production over the years at around 37-40 kilos (81-88 pounds).

US intelligence estimates are of 40-50 kg.

The official was unsure whether the North was rejecting sampling because it has something to hide or because this would be too great a concession at present.

He said a compromise would be hard to find, especially now the North has publicly announced its position.

In its Tuesday statement the North complained that the US pressurised it "to accept such inspection as scouring any place of the (North) as it pleases to collect samples and measure them."

But the official said North Korea would find it hard to quit the six-party process altogether because it would have too much to lose.

The North has so far received only half of the one million tonnes of fuel oil or equivalent assistance promised by negotiating partners in return for disablement.