North Korea's first steps to roll back a nuclear arms program launched about 40 years ago are going well, a U.S. official said on Tuesday after visiting the North's plutonium-producing atomic complex.

Destitute North Korea struck a deal with regional powers last month to disable its Soviet-era nuclear complex in exchange for aid and an end to its international ostracism.

I think we are off to a good start, U.S. State Department official Sung Kim said at Incheon airport near Seoul, according to a pool report. Kim was with a team of U.S. nuclear specialists who arrived in North Korea last week.

He said steps had been taken to reverse the operations at all three of the key facilities -- the North's ageing reactor, a plant that produces nuclear fuel and another that turns spent fuel into arms-grade plutonium.

Our North Korean colleagues have actually done a considerable amount of preparatory work on all three facilities.

Kim said he believed disablement at one facility would be completed this week. The team wants irradiated fuel rods removed from the reactor, which experts said would halt its operations and could pave the way for further decommissioning steps.

The deal requires North Korea to disable the three plants by the end of 2007, provide a list of its nuclear arms activity, account for all its fissile material and answer U.S. suspicions of having a clandestine program to enrich uranium for weapons.

In Vienna, the International Atomic Energy Agency said IAEA personnel were monitoring the disabling program. North Korea kicked out IAEA monitors in 2002, but they re-entered in July to verify the shutdown of the Yongbyon nuclear complex.

Under the deal it reached with China, Japan, Russia, South Korea and the United States, the energy-starved North is to receive 1 million tonnes of heavy fuel oil or equivalent aid.

The United States will also move towards taking North Korea off a U.S. terrorism blacklist.

U.S. officials estimate the North has about 50 kg (110 lb) of plutonium. Proliferation experts say that is enough for six to eight bombs.

A State Department spokesman in Washington said disablement had started on Monday. North Korea, which conducted its first nuclear test in October 2006, shut the three facilities in July.

Experts say that, although the disablement steps are reversible, they would prevent North Korea from going back to producing any more plutonium for about a year.

The North set up its nuclear research facility at Yongbyon, about 100 km (60 miles) north of Pyongyang, in the mid-1960s with help from Cold War communist allies. It began building its own reactor there in the early 1980s, experts said.

The North has frozen its facilities before but never before taken significant steps to reverse its rudimentary nuclear arms program, a diplomatic mainstay for wringing concessions from the outside world.

(additional reporting by Mark Heinrich in Vienna, editing by Jonathan Thatcher and Roger Crabb)

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