UN nuclear inspectors have arrived in Pyongyang, for talks on the closure of North Korea's main nuclear reactor.
This is the first time since 2002 that inspectors have visited
Pyongyang agreed to shut the Yongbyon plant under a deal reached in February.
The head of the team, Olli Heinonen, said he was optimistic about the trip, but admitted he was unclear whether he would be allowed to visit Yongbyon.
South Korea has said it will resume food aid to the impoverished North, now that a dispute over frozen North Korean funds has finally been solved.
The deliveries of rice to the country, which is still struggling with food shortages, are due to start on 30 June.
A four-member team from the International Atomic Energy Agency (IAEA) arrived in Pyongyang on Tuesday morning - the first time they have been allowed into the country since 2002.
Mr Heinonen said they would be discussing procedures for IAEA inspectors to monitor the shut-down and sealing of Yongbyon.
Before leaving for Pyongyang, he told reporters he was not sure whether he would get direct access to Yongbyon or how long it would take North Korea to shut down the facility.
N KOREAN BANKING ROW
13 Feb 2007: Landmark nuclear disarmament deal agreed at six-party talks
March: N Korea refuses further progress on the deal until it has access to funds frozen in a Macau bank
April: N Korea misses a deadline to "shut down and seal" its Yongbyon nuclear reactor
25 June: Pyongyang finally says it has the funds
26 June: IAEA nuclear inspectors arrive in Pyongyang
But he still expressed optimism. "I think [North Korea] will now do what they have been asked to do," he said.
The visit comes after a long delay in the implementation of the nuclear disarmament deal agreed in February, which was caused by a row over North Korean funds that were frozen in a Macau bank under US-instigated sanctions.
The North refused to make any progress on the agreement until it had access to the money.
On Monday, Pyongyang said it had finally received its $25m (£12.5m) funds, and confirmed it would proceed with its agreement to shut Yongbyon.
Now the process is back on track, chief US negotiator Christopher Hill believes Pyongyang could shut its reactor within three weeks.
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"
The closure of Yongbyon would be the trigger for discussing wider issues, such as economic assistance and peace talks, Mr Hill told reporters after a recent surprise trip to the North.
He also held out the prospect of a meeting that would include US Secretary of State Condoleezza Rice and North Korea's foreign minister later this summer.
"I can't say whether it's at the Asean regional forum or just before or just after, but some time in that time frame," Mr Hill said, referring to the Association of East Asian Nations summit in Manila on 2 August.
But he stressed that further progress depended on North Korea coming clean on a number of issues - including whether it had stockpiles of enriched uranium.
South Korea's Unification Minister Lee Jae-joung announced that the first shipment of 3,000 tonnes of rice aid would leave for the North's port of Nampo on Saturday.
He said the entire shipment of 400,000 tonnes of food would take about six months to complete.
Mr Hill believes the next few weeks are critical to progress
The decision was made "in consideration of the request of the public and international community on a humanitarian issue", he said.
South Korea suspended food aid shipments a year ago after its northern neighbour test-fired long range missiles, before then testing a nuclear weapon in October.
Seoul had intended to resume the shipments in April after February's deal, but put them on hold while the nuclear issue was in doubt.
"The implementation of the 13 February deal was necessary for this," Mr Lee said.