China Thursday welcomed North Korea's agreement to freeze nuclear activities in return for massive US food aid, a deal that raised cautious hopes of eased tensions under Pyongyang's new young leader.

South Korea and Japan also hailed Pyongyang's commitment to suspend its uranium enrichment programme along with nuclear and long-range missile tests, and to let UN nuclear inspectors monitor the deal.

The announcement follows the death in December of longtime leader Kim Jong-Il and the transition to his untested son Jong-Un.

The deal could boost the son's prestige in the run-up to a major celebration next month, marking 100 years since the birth of the Kim dynasty's late founding leader Kim Il-Sung.

The breakthrough followed US-North Korean talks in Beijing last week, the first under the new regime.

China, the North's sole major ally and economic prop, welcomed the warmer relations between North Korea and its longtime foe the United States.

"China is willing to work with relevant parties to continue to push forward the six-party talks process, and play a constructive role to realise long-term peace and stability on the Korean peninsula and northeast Asia," said foreign ministry spokesman Hong Lei.

The six-nation nuclear disarmament talks have been stalled for some three years. But the two Koreas, Japan, China, Russia and the United States have been talking for months about ways to revive them.

The disclosure in November 2010 of the enrichment programme, which could give the North a second path to an atomic bomb, lent urgency to the diplomacy.

South Korea, whose relations with its neighbour have remained icy under the new leadership, also backed the agreement disclosed simultaneously by the US and North Korea on Wednesday night.

Russia's foreign ministry welcomed the moratorium on nuclear testing and uranium enrichment.

Japan's Foreign Minister Koichiro Gemba said the deal was "an important step" but called for concrete action. Tokyo still wants "the complete and verifiable denuclearisation of the Korean peninsula", he said.

That is also the stated goal of the six-party talks which have dragged on since August 2003. The North angrily quit the forum in April 2009 and staged its second atomic weapons test a month later.

Analysts said Wednesday's deal could help revive the talks, but many remain sceptical that the North will ever abandon its nuclear weaponry.

"At this point the best that can be done is to freeze the nuclear programme," said Peter Beck, Korea representative for the Asia Foundation.

"For now, the agreement is a welcome development. Talking is better than not talking and a freeze is better than an unfettered nuclear programme."

Kim Jong-Un, Beck told AFP, appeared to have decided that "feeding his people is seen as more important than expanding nuclear facilities".

The United States has pledged 240,000 tonnes of food designed for young children and pregnant women and difficult to divert to the North's military.

The North has suffered persistent severe food shortages since a 1990s famine, but still spent massively on a nuclear programme thought to have produced enough plutonium for six to eight weapons.

It says it needs such a deterrent against US hostility.

Washington "reaffirms that it does not have hostile intent" towards the North and is prepared to improve the relationship, said State Department spokeswoman Victoria Nuland.

Pyongyang said it would let the International Atomic Energy Agency monitor the suspension of uranium enrichment. Agency chief Yukiya Amano called this "an important step forward" and said his inspectors were ready to return.

North Korea's chief nuclear negotiator Ri Yong-Ho could visit the US to attend a security forum at Syracuse University in New York State, South Korea's Yonhap news agency quoted a diplomatic source as saying.

The agency, in a report from Washington, said the US has not made yet made a final decision on whether to issue a visa for him.

US administration officials, already under fire in an election year from Republican critics, were cautious about prospects.

"Today's announcement represents a modest first step in the right direction," said Secretary of State Hillary Clinton. "We of course will be watching closely and judging North Korea's new leaders by their actions."