A ship carrying the first delivery of fuel to North Korea, under a key nuclear disarmament deal, has left a South Korean port.
The fuel shipment is part of an international disarmament deal
The ship, carrying 6,200 tons of oil, is expected to arrive in North Korea on Saturday.
Pyongyang has indicated it may begin the process of closing its Yongbyon nuclear reactor once the aid arrives.
A UN nuclear team is also due to arrive in Pyongyang on Saturday, to start monitoring the reactor's shutdown.
International Atomic Energy Agency chief Mohamed ElBaradei said he expected the operation "to move smoothly".
"We already have an agreement on how to go about it," he said.
The initial shutdown would start early next week, he said, and cameras and monitoring equipment should be installed within a month.
But Mr ElBaradei warned that disarming North Korea would be a long process.
N KOREA NUCLEAR DEAL
N Korea to "shut down and seal" Yongbyon reactor, then disable all nuclear facilities
In return, will be given 1m tons of heavy fuel oil
N Korea to invite IAEA back to monitor deal
Under earlier 2005 deal, N Korea agreed to end nuclear programme and return to non-proliferation treaty
N Korea's demand for light water reactor to be discussed at "appropriate time"
"We should not delude ourselves," he said. "This has been a problem for over 15 years, the Korean nuclear issue, and it will take time to have a comprehensive solution."
The next step would depend on the outcome of six-way international talks due next week, he added.
Multilateral talks involving North Korea and its dialogue partners - China, Japan, Russia, South Korea and the US - to discuss futher progress towards ending Pyongyang's nuclear programme are expected to resume in Beijing next week.
North Korea agreed in February to shut down its reactor, in return for energy aid.
The oil delivery is the first shipment of the one million tons of fuel aid that Pyongyang has been promised in return for disarming.
Implementation of the deal was initially delayed by because of a financial dispute, but this has now been resolved and progress appears to be being made.
Last month an international UN team was given access to the Yongbyon plant for the first time since 2002.