(Adds details on working groups, paragraphs 17,18)

By Lindsay Beck

BEIJING, March 13 (Reuters) - The head of the U.N. nuclear watchdog arrived in North Korea on Tuesday on a landmark visit, hopeful of making progress on closing its atomic facilities, but U.S. officials sounded a more cautious note.

Inspectors from the International Atomic Energy Agency have not visited North Korea since the isolated and impoverished state expelled the group in late 2002 as a disarmament deal fell apart. It withdrew from the nuclear Non-Proliferation Treaty days later.

Now, as part of a new accord reached in February, the North has agreed to admit the watchdog, which will play a key role in verifying whether it meets a commitment to shut down the Yongbyon reactor at the heart of its nuclear programme.

"I hope we should be able to make some progress," IAEA head Mohamed ElBaradei told reporters before leaving Beijing.

He hoped his agency could "work closer with the DPRK after many years of estrangement", he said, referring to the North by its official name, the Democratic People's Republic of Korea.

Under the February deal, cut at six-party talks in Beijing that group the two Koreas, the United States, China, Japan and Russia, North Korea agreed to shut Yongbyon by mid-April in return for an infusion of energy aid and security assurances.

"This is an important part of the six-party talks' implementing of the initial steps," ElBaradei said of his trip.

"I think obviously these initial steps will be important, significant in fact, in moving the six-party talks forward."

One U.S. official expressed confidence that North Korea would make good on its pledge to shut down Yongbyon.

"It can be done in a short period of time (and) we're reasonably confident they will take that step," the official said.

WORKING GROUPS

But others said it was still too soon to make predictions.

"My understanding is that some of what we have seen in recent weeks is maintenance and the like, but we'll have a better idea after ElBaradei visits this week," a second U.S. official said.

The six countries are preparing to meet again from March 19 in Beijing, the centre of a renewed flurry of diplomacy on North Korea.

The chief U.S. envoy to the talks, Christopher Hill, will arrive in Beijing on Wednesday, where he is to attend working group sessions that were agreed as part of the February deal.

He may also meet ElBaradei, who is expected to return from Pyongyang late on Wednesday.

South Korean negotiator Chun Yung-woo is also due in Beijing for separate working group meetings on Thursday on providing aid and energy to the North.

A working group on denuclearisation chaired by Chinese chief envoy Wu Dawei would meet on Saturday, Chinese Foreign Ministry spokesman Qin Gang said.

"We believe the goal for the five working groups is to implement the initial actions of the joint statement to lay a foundation for moving forward the six-party talks," Qin told a news conference.

Despite activity aimed at implementing the February agreement, analysts say achieving the ultimate goal of seeing the notoriously unpredictable North scrap its nuclear programmes was still uncertain.

"Because of the complexity of the North Korea nuclear issue, thoroughly realising the denuclearisation of the Korean peninsula still requires the long-term, joint efforts of all of the parties," China's Xinhua news agency said in a commentary. (Additional reporting by Carol Giacomo in Washington)

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