SEOUL - US NEGOTIATOR Christopher Hill on Tuesday predicted tough negotiations with North Korea as he prepared to leave for Pyongyang to try to save a crumbling nuclear disarmament deal.

'I would say we are in a difficult and very tough phase of negotiations,' he told reporters after talks with his South Korean counterpart Kim Sook.

The communist North, locked in a dispute over verification of its nuclear declaration, has announced it will begin restarting its plutonium reprocessing plant at Yongbyon and has barred UN atomic inspectors from the building.

Mr Hill leaves on Wednesday for Pyongyang, his third trip in 16 months. He will meet the communist state's nuclear negotiator Kim Kye Gwan to try to overcome the most serious hurdle so far to implementing the February 2007 six-party deal.

The assistant secretary of state struck a cautious note on arrival earlier.

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

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.'

Mr Hill reiterated that Washington is ready to remove Pyongyang from a terrorism blacklist, which blocks some US and multilateral aid, when verification procedures are agreed to.

The ageing Yongbyon complex is at the heart of North Korea's decades-old nuclear ambitions, which culminated in a weapons test in October 2006.

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

However, Washington refuses to delist the North until it accepts procedures for outside verification of the declaration it submitted in June.

The North says verification is not part of this stage of the deal and accuses the United States of violating its dignity by seeking 'house searches', such as in Iraq.

Asked about the North's objections to the proposals on site inspections, Mr 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.'

Mr Hill will travel to China and Japan after North Korea. Russia is the sixth member of the negotiating forum.

The North 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 on 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 on Monday, said hardliners such as US Vice-President Dick Cheney were largely to blame for the current deadlock.

It said they were 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 were excessive, saying six-nation delegation chiefs in July had agreed to verification principles. -- AFP