North Korea has shut down all five nuclear facilities at its main Yongbyon complex, International Atomic Energy Agency head Mohamed ElBaradei says.
The closure of Yongbyon is the first step in a long process, Mr Hill warns
IAEA monitors in North Korea had verified the four new closures, Mr ElBaradei told journalists in Malaysia.
North Korea shut down its sole working reactor at Yongbyon on Saturday.
After a day of international talks in Beijing, a South Korean official said Pyongyang had agreed to disable all its nuclear facilities by the year's end.
Chun Yung-woo, Seoul's envoy at the talks, said North Korean negotiator Kim Kye-gwan also indicated Pyongyang's willingness to declare all aspects of its nuclear programme.
Chief US negotiator, Christopher Hill, was unable to confirm the North's offer, but he did describe the talks as open and substantive, saying there had been "a lot of agreement around the table about what needs to be done in this phase".
He cautioned against premature optimism, saying "there has to be a follow-on phase... and that is the actual abandonment of the fissile material and explosive devices on the DPRK's (North Korea's) side".
Wednesday was the first of two days of talks involving China, Japan, South Korea and Russia, as well as the US and North Korea.
"We have verified that all five nuclear facilities have been shut down and that appropriate measures have been put in place, including sealing some of these facilities," Mr ElBaradei told journalists earlier in Kuala Lumpur.
"We expect that in the next few weeks we will continue to apply the necessary monitoring and verification measures," he said.
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"
Facilities shut down include construction sites for a reactor and a fuel reprocessing site.
The Yongbyon closures are the first step in a deal agreed in February 2007, under which North Korea is to receive a total of one million tons of energy aid if it ends its nuclear programme.
Analysts say that while the Yongbyon closures are an important step, persuading North Korea to fully disclose all of its nuclear facilities and agree to their being disabled is likely to be a long and difficult process.
One hurdle is the US allegation that North Korea - which carried out its first nuclear test in October 2006 - has a secret uranium enrichment programme. Pyongyang denies this.
"Uranium enrichment is an ongoing issue and, believe me, we are working on it," Mr Hill said earlier on Wednesday.
Mr ElBaradei emphasised that full transparency from Pyongyang was the key.
"The more transparency we get, the quicker we will be able to verify that everything in the DPRK has been declared," he said.