"We took a long time to get these first steps and we have really a lot work to do now, but I think we're off to a good start," Christopher Hill, an assistant US secretary of state, told reporters in the South Korean capital, Seoul.
"North Korea has only one card in hand and that is its nuclear programme
Rahy, Tehran, Iran
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Hill said the disablement of the North's nuclear facilities could be completed by the end of the year, and he expected a complete declaration of all North Korea's atomic programmes within months.
But he added there were still a number of steps left before they could reach complete denuclearisation, and he expected that the long delays in shutting down the Yongbyon plant meant there could still be problems ahead.
Hill's comments came as South Korea sent a second shipment of oil to the North, in line with six-nation disarmament-for-aid deal signed in Beijing in February.
The tanker carrying 7,500 tonnes of oil left Ulsan in the South for the North Korean port of Nampo.
The first shipment of oil arrived in the North on Saturday, shortly after which North Korean officials announced they had shut down the Yongbyon reactor.
On February 13, 2007, at six-nation talks in Beijing, North Korea agreed to:
Start shutdown of main Yongbyon nuclear reactor facility within 60 days of deal
Allow UN nuclear inspectors entry for all monitoring and verification
Discuss list of all nuclear programmes and materials including plutonium extracted from fuel rods
Declare all nuclear programmes and disarmament of all existing nuclear facilities
Begin talks on normalising diplomatic ties with the US and Japan, and resume high-level talks with South Korea
In return US, Russia, China, Japan and South Korea promise initial shipment of 50,000 tonnes heavy fuel oil within initial phase
The five nations agreed to establish working groups for initial and full implementation of action plan
Additional aid up to the equivalent of 1m tonnes of heavy fuel oil to be delivered to North Korea upon compliance
The two shipments are part of an initial 50,000 tonnes of oil that was pledged to the North in return for closing the reactor.
Pyongyang is to receive a further 950,000 tonnes once it has shut down its nuclear weapons programme.
The shutdown of the Yongbyon reactor, located about 100km north of Pyongyang, is the first step taken by North Korea to scale back its nuclear programme since it expelled UN nuclear inspectors in 2002.
On Saturday a 10-member team of inspectors from the International Atomic Energy Agency returned to North Korea on Pyongyang's invitation.
Under the six-nation deal, aside from energy aid the North will receive security assurances and the dropping of international sanctions among other benefits if it completely and verifiably scraps its nuclear arms programme.
"With complete denuclearisation, everything is going to be possible," Hill told reporters in Seoul.
He and other negotiators in the six-party talks are expected to travel to Beijing this week to discuss plans for pushing the disarmament process forward.