(Adds Japan comment, paragraphs 11-12)

By Lindsay Beck

BEIJING, March 12 (Reuters) - The head of the U.N. nuclear watchdog said on Monday that moving forward to inspect and close facilities behind North Korea's nuclear weapons programme would be complex as the two sides seek to rebuild severed ties.

International Atomic Energy Agency director Mohamed ElBaradei was in Beijing en route to North Korea, where he is to negotiate the return of agency inspectors as part of a Feb. 13 accord.

That pact aims to wind down North Korea's nuclear weapons ambitions in exchange for aid and security assurances.

"It is going to be a very incremental process," he told reporters on arrival in Beijing. "There's a lot of confidence that needs to be built."

IAEA inspectors have not visited the isolated North since 2002, when Pyongyang expelled them as a previous disarmament deal ruptured. Days later, North Korea announced its "automatic and immediate" withdrawal from the Nuclear Non-Proliferation Treaty.

"We need a lot of bridges to build, confidence to re-establish," ElBaradei said.

A shutdown of North Korea's Yongbyon nuclear plant by mid-April is the centrepiece of last month's accord reached in six-party talks grouping the two Koreas, Japan, Russia, the United States and host China.

"I hope we can agree with the DPRK (North Korea) to get our inspectors back in time to implement the agreement of the six-party talks," he said.

But he was not certain the IAEA and Pyongyang could agree on how to proceed in time to meet the 60-day timeframe for the shutdown.

MOVE FORWARD

"I'd like this trip at least to establish the framework and then gradually move forward," he said of North Korea's denuclearisation. "It is in their interests obviously to keep to that deadline, but we'll see."

Japan's top government spokesman said a positive outcome to ElBaradei's trip was vital to convincing the world that North Korea was sincere about scrapping its nuclear arms.

"It is indispensable (for North Korea) to secure confidence through the IAEA's activities, and we of course hope that their activities will produce a breakthrough," the spokesman, Yasuhisa Shiozaki, told reporters

ElBaradei also wanted to discuss North Korea's re-entry into the IAEA, which oversees global nuclear safeguards, including the Non-Proliferation Treaty.

"I hope we will be able to agree on modalities to normalise the relationship with the IAEA and hopefully for the DPRK to come back as a full member of the agency."

North Korea announced in 2005 it had nuclear arms and in 2006 it test-detonated its first nuclear device, drawing U.N. financial and arms sanctions.

ElBaradei was due to meet China's chief envoy to the six-party talks, Vice Foreign Minister Wu Dawei, on Monday, and leave for Pyongyang on Tuesday, an IAEA official said.

It was still unclear who ElBaradei would meet in North Korea, but he said he was hopeful at least some headway could be made during the landmark visit.

"As long as we are talking, as long as we are making steady progress, I am quite pleased," he said. (Additional reporting by Chris Buckley in Beijing in Teruaki Ueno in Tokyo)

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