The first shipment of fuel oil to North Korea – part of an aid-for-disarmament pact agreed in February – has left port on a tanker from South Korea.
The delivery comes as the first part of an initial 50,000 tons of heavy fuel oil pledged to the North in return for shutting down its plutonium-producing nuclear reactor at Yongbyon.
The first delivery of some 6,200 tones of oil is expected to arrive in North Korea’s post of Sonbong at the weekend.
Last week North Korea promised to suspend operations at its nuclear facilities as soon as the first shipment of heavy fuel arrives.
Inspectors from the International Atomic Energy Agency (IAEA), the UN’s nuclear watchdog, are expected to arrive in North Korea at the weekend to monitor and verify the shutdown of Yongybyon.
Mohamed ElBaradei, the head of the IAEA, said he was optimistic that the latest developments were a sign of progress towards solving the nuclear row after months of delays.
"I expect that operation to move smoothly," the chief nuclear inspector told reporters in Seoul.
"I am quite optimistic that this is a good step in the right direction."
Further shipments are expected to follow in the coming days although SK Corp, South Korea’s top oil refiner, refused to confirm the schedule, citing sensitivity of the matter.
A company spokeswoman said: "We cannot comment on this issue because it is government related."
On Tuesday, SK Corp provisionally booked a 47,000 tones tanker for loading on July 27 but the booking was said to have fallen through.
Under the six-nation deal agreed in February in Beijing, Pyongyang will receive oil when it starts shutting down the Yongbyon complex.
South Korea has agreed to bear the cost of the oil and organize the shipments.
Under the Beijing agreement North Korea will get an additional 950,000 tons of energy aid if it disables all its nuclear facilities.
However, ElBaradei warned that beyond the initial moves, the eventual entire abandonment of the North’s nuclear facilities would be a "long process" that would depend on progress at the six-nation disarmament talks.
"We should not delude ourselves," he said. "It will take time to have a comprehensive solution."
Also on Thursday Lee Jae-joung, South Korea’s unification minister, said that once the fuel began to arrive the ball was in North Korea’s court.
The shutdown of Yonbyon, he said, was "absolutely necessary" for the sake of lasting peace on the divided Korean peninsula.