UN inspectors have begun verifying that North Korea has really closed down its nuclear reactor at Yongbyon, the top US nuclear envoy has said.
A 10-member IAEA team are overseeing the process at Yongbyon
Christopher Hill's comments come a day after Pyongyang told Washington it had shut the reactor.
North Korea's announcement was welcomed by both the US and South Korea.
North Korea agreed to close the reactor in February in return for economic aid. Under the deal, Pyongyang got its first heavy fuel oil shipments on Saturday.
The nuclear team from the International Atomic Energy Agency (IAEA) arrived in North Korea on Saturday to start the Yongbyon inspection.
If confirmed, the shut-down would be the first stage in disabling the North's nuclear programme.
It could take the IAEA at least a day to verify the closure, because there are five sites to inspect within the complex, Mr Hill said on Sunday.
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"
But the envoy, who is touring the region ahead of six-nation talks on North Korea's nuclear programme, said he was confident the North Korean nuclear shut-down had begun.
"I think we have every reason to believe they have indeed started the shut-down," he said.
The IAEA's 10-member inspection team is to decommission and seal equipment at the reactor and plutonium reprocessing plant.
It may take the inspection team up to three weeks to complete the task, says the BBC's Kevin Kim in Seoul.
On Saturday, Washington said it had been informed by North Korea of the Yongbyon shut-down.
A North Korea diplomat later confirmed that the closure had taken place.
"Immediately after the arrival of the first heavy fuel oil, the facilities were shut down and the (IAEA) personnel will verify that," Kim Myong Gil, an official at the North's mission to the UN, told the Associated Press news agency.
North Korea was promised heavy fuel oil in exchange for the Yongbyon shut-down at six-party talks in Beijing in February.
The deal was delayed amid a wrangle over North Korean funds frozen in a Macau bank account.
But the first ship of oil eventually arrived in the North Korean port of Sonbong on Saturday, loaded with 6,200 tonnes of heavy fuel oil.
Mr Hill has stressed the need to press ahead with negotiations
Mr Hill has emphasised that the closure of the Yongbyon reactor is only the first step in decommissioning North Korea's nuclear programme.
He has said he expects a full list of the country's nuclear facilities within months - as agreed in the February deal.
Arriving in the South Korean capital Seoul on Sunday to discuss the six-nation talks, set to begin in Beijing on Wednesday, he emphasised the need to press ahead with negotiations with Pyongyang.
"If we don't take these steps a little more quickly than we have taken that first step, then we are going to fall way behind again," Mr Hill told reporters.
"I think it's very important that the six-party process work closely together and quickly together, so that by the end of the year, we can see some real progress and get on to what I'd like to call 'the end game' in 2008," he added.
The participating countries - South and North Korea, Russia, Japan, the US - are expected to negotiate the details of the next phase of the North's decommissioning process, namely the declaration of its nuclear programme and disabling the facilities.
North Korea tested an atomic bomb for the first time last year, and has repeatedly said it needs nuclear weapons to fend off a US attack.