US negotiator Christopher Hill arrived Tuesday in Seoul en route to Pyongyang but was cautious about the prospects of saving a North Korean nuclear disarmament deal that appears close to collapse.

Hill's rare trip to the North Korean capital to discuss the six-nation deal comes after the communist state announced it would begin restarting its plutonium reprocessing plant within a week.

"We'll see what happens when I go up to the DPRK (North Korea) tomorrow," he told reporters at Incheon airport west of the South Korean capital.

"Let's see if we can make some progress on phase two."

The current phase of the six-nation deal is deadlocked over US-led demands for thorough outside inspections to verify the nuclear inventory which the North declared in June.

Last week the North announced it would begin restarting the plant and ordered UN atomic inspectors to quit the building at its Yongbyon complex.

Asked if it was positive that Pyongyang had invited him, the veteran negotiator said only: "I'll be able to answer that in a couple of days."

He said the two sides had had some discussions through the North's UN mission in New York on the dispute.

"We thought it would be useful to try to have those discussions in Pyongyang. That's why I'm going."

Hill reiterated that Washington is ready to remove Pyongyang from a terrorism blacklist when verification procedures are agreed.

"Let's see if we can come up with measures that will allow us to verify their declaration," he said.

The aid-for-disarmament deal reached in February 2007 has hit several roadblocks before but this is seen as the most serious.

The North shut down Yongbyon in July 2007 and began disabling the plants in November that year. In return it was to receive one million tons of fuel oil or equivalent energy aid, and the US was to remove it from the blacklist which blocks some foreign aid.

However Washington refuses to delist the North until it accepts the verification protocol.

The North says verification is not part of this stage of the deal and accuses the US of violating its dignity by seeking "house searches" as in Iraq.

Asked about the North's objections to the proposals on site inspections, Hill said: "I know they are reluctant. Let's see what they say. Let's sit down and have a conversation and see if we can resolve this matter."

He was to hold talks later Tuesday with his South Korean counterpart Kim Sook and will go on to China and Japan after North Korea. Russia is the sixth member of the negotiating forum.

The North, which tested an atomic weapon in October 2006, is estimated to have produced enough plutonium for around six bombs before Yongbyon was shut down.

Analysts believe it could produce enough material for one more bomb if it resumes reprocessing spent reactor fuel rods.

Japan said Tuesday it will extend sanctions against North Korea for another six months.

"North Korea has stopped the denuclearisation process while moving to reactivate its nuclear plant," said Foreign Minister Hirofumi Nakasone.

"Under the current situation, we will have no other option but to extend the sanctions."

The New York Times, in an editorial Monday, said hardliners such as US Vice President Dick Cheney are largely to blame for the current deadlock.

It said they are trying to make the North accept a verification programme "that only a state vanquished in war might accept."

Deputy State Department spokesman Robert Wood on Monday denied US demands are excessive, saying six-nation delegation chiefs in July had agreed verification principles.

"And as I said, we're talking about a standard verification package. This is not onerous. It's not unusual in terms of trying to verify activities that may have taken place," Wood said.