"Our sense is that we will be down to a matter of weeks," Hill told reporters in Beijing, when asked about the expected timeframe for the North to disable its main nuclear reactor at Yongbyon.

"The disablement phase can actually be fairly quick."

South Korea also said earlier Monday it expected North Korea to start shutting its Yongbyon reactor in two to three weeks.

The US and South Korean remarks came after an announcement on Saturday by the North that it was ready to welcome inspectors from the International Atomic Energy Agency, the UN's nuclear watchdog, back into the country.

The North made the invitation as a banking dispute with the United States that had blocked a February disarmament deal appeared to be almost over.

The deal -- brokered between the two Koreas, the United States, Russia, China and Japan -- was meant to see the North close Yongbyon and allow the return of IAEA inspectors, in exchange for 50,000 tons of heavy fuel oil.

Under a second phase of the pact, the North, which has worked for decades to develop a nuclear bomb, would disable all its atomic programmes in return for another 950,000 tons of oil or equivalent aid, and major diplomatic benefits -- including the establishment of diplomatic relations with the United States.

"Now (we are) getting prepared for the shutdown of the nuclear facility, we will want to continue to implement (the six-nation agreement) and do it with some pace," said Hill, who met with Chinese nuclear envoy Wu Dawei on Monday.

The February accord was put on ice due to North Korea's insistence that it first receive 25 million dollars that had been frozen in a Macau bank since 2005 over US allegations of counterfeiting and money laundering.

It unblocked the money in March in an effort to kick-start the disarmament process but, due to a myriad of complications, the North could not retrieve the money until a deal was worked out for the funds to be transferred via Russia.

Hill said he had not yet had confirmation that the money had been returned to North Korea, but "our understanding is that the matter has been resolved."

"We were aware it is in the Russian banking system. It will be there (in North Korea's account) soon, if it isn't already," Hill said.

Hill said he believed South Korea would this week make the first order of heavy duty fuel oil to be delivered to the North.

The US envoy was was set to travel to Seoul later in the day for talks on pushing the six-party disarmament process forward after a four-month stalemate. He will go on to Tokyo on Tuesday.

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