* N.Korea move could open door to new 6-party disarmament talks
* U.S. agrees to food aid in confidence building measure
* Dramatic step follows December death of Kim Jong-il (Adds Ban Ki-moon comments)
By Andrew Quinn
WASHINGTON, Feb 29 (Reuters) - North Korea agreed on Wednesday to stop nuclear tests, uranium enrichment and long-range missile launches, and to allow checks by nuclear inspectors, in an apparent policy shift that paves the way for resuming long-stalled disarmament talks.
The surprise breakthrough, announced simultaneously by the U.S. State Department and North Korea, also includes U.S. food aid for the impoverished state and makes possible the resumption of six-nation nuclear negotiations with Pyongyang. The news followed bilateral diplomatic talks in Beijing last week.
"These are concrete measures that we consider a positive first step toward complete and verifiable denuclearization of the Korean peninsula in a peaceful manner," White House spokesman Jay Carney said.
Analysts cautioned that Pyongyang had backtracked repeatedly on past deals, but the moves by North Korea mark a sharp change in course, at least outwardly, by North Korea's reclusive leadership after the death in December of veteran leader Kim Jong-il.
One senior U.S. official said the move "unlocked" an impasse over the six-party talks, but that follow-through would require persistence and patience.
"We believe that its important to translate this initial sign of Pyongyang's seriousness of purpose into substantive and meaningful negotiations on denuclearization that get at the entirety of the North's nuclear program," the official said.
The State Department said that in return, the United States was ready to go ahead with a proposed 240,000 metric-tonne food aid package requested by North Korea and that more aid could be agreed to based on continued need.
Along with halting weapons activities, North Korea said it would permit nuclear inspectors from the U.N.'s International Atomic Energy Agency to visit its Yongbyon nuclear complex to verify the moratorium on uranium enrichment has been enforced.
"The DPRK, upon request by the U.S. and with a view to maintaining positive atmosphere for the DPRK-U.S. high-level talks, agreed to a moratorium on nuclear tests, long-range missile launches, and uranium enrichment activity at Yongbyon and allow the IAEA to monitor the moratorium on uranium enrichment while productive dialogues continue," North Korea's official KCNA news agency said.
North Korea is known formally as the Democratic People's Republic of Korea (DPRK).
Secretary of State Hillary Clinton told a Congressional panel the North Koreans had made "a modest first step in the right direction," but noted that Washington continued to have profound concerns over a range of North Korean activities.
The IAEA, which withdrew its inspectors from North Korea in 2009, said it was ready to return, calling the moratorium deal "an important step forward".
South Korea and Japan both welcomed the announcement, with the Foreign Ministry in Seoul saying it could form the basis for a broader agreement on North Korea's nuclear program..
U.N. Secretary-General Ban Ki-moon said he hoped North Korea would move towards a "verifiable denuclearization of the Korean peninsula".
"The Secretary-General also stresses the urgency of meeting the humanitarian needs of the most vulnerable people in DPRK," his spokesman Martin Nesirky said in a statement.
The U.S. decision to resume food aid was a gesture toward Pyongyang, which has sought international help to cope with chronic food shortages.
The United States halted food aid to North Korea in 2009 amid a dispute over transparency and monitoring, compounding problems that have followed a crippling famine in the 1990s that killed an estimated one million people.
Under the new aid proposal, the United States is ready to provide emergency nutrition, including corn-soy blend, vegetable oil and therapeutic foods to help fight chronic malnutrition among young children, pregnant women and other vulnerable people, U.S. officials said.
The surprise announcement was a step forward for Washington's campaign to rein in renegade nuclear programs around the world and comes as the Obama administration steps up pressure on Iran over its nuclear ambitions, which western governments fear are aimed at producing nuclear weapons.
It also comes several weeks before U.S. President Barack Obama visits Seoul for a nuclear security summit in March.
Analysts called the deal an important preliminary step and said the return of IAEA inspectors would give the international community an important window into North Korea's nuclear work.
"This puts an element of control back on the North Koreans' nuclear development program as well as their existing capabilities that we have not had for almost four years," said Jack Pritchard, a former U.S. negotiator with North Korea who heads the Korea Economic Institute.
But Pritchard said he believed it was unlikely that Pyongyang's young and untested new leader Kim Jong-un was ready to comply with demands that he scrap the entire nuclear program.
"How does a 28-year-old give up the only legitimate piece of leverage that he has in dealing with the superpowers to preserve the survivability of his regime? He's not going to do that," Pritchard said.
In Congress, the powerful Republican head of the House Foreign Affairs Committee said Pyongyang would likely continue its clandestine nuclear program "right under our noses".
"We have bought this bridge several times before," Rep. Ileana Ros-Lehtinen said in a statement.
NEW LEADER AT THE TOP
The announcement followed talks between the United States and with North Korea last week in Beijing, the first such meeting since Kim Jong-un succeeded his father as leader of the communist state two months ago.
Bruce Klingner, a Korea analyst at the Heritage Foundation, said the move did not necessarily represent any fundamental change by Pyongyang, noting that it tracked a draft deal U.S. diplomats were nearing at the time of Kim Jong-il's death.
"This is the first step in a very long road," he said, saying it may simply provide the framework for additional meetings between the United States and North Korea to haggle over an agenda for any broader nuclear talks.
U.S. officials said it was up to North Korea to contact the IAEA to discuss resumed inspections, and that they expected tough negotiations over the sequence of steps to be taken.
"I wouldn't look for people to be in motion right away here," one official said.
North Korea agreed to curtail its nuclear activities under a an aid-for-denuclearization agreement reached in September 2005 by six-party talks bringing together North and South Korea, China, Japan, Russia and the United States.
Under the agreement, the North agreed to abandon its nuclear programs in exchange for economic and diplomatic incentives to be provided by the other parties involved in the negotiations.
But the embryonic deal was never fully implemented.
Instead, the North held two nuclear test blasts -- in 2006 and 2009 -- and later disclosed a uranium enrichment program, giving it a second path to obtaining fissile material for bombs, in addition to its long-standing program of producing plutonium.
The United States, South Korea and their allies had been sceptical of North Korea's assertions that it stood ready to return to the six-party talks, and U.S. officials said they would insist on demonstrable progress.
"The truth is we've been around the six-party block before. It has a history of ups and downs, sometimes more downs than ups," one of the U.S. officials said.
"We can't allow the same patterns of the past to repeat themselves." (Reporting by Andrew Quinn; Editing by Anthony Boadle and Mark Bendeich)