North Korea had begun disabling Yongbyon before a recent dispute
UN monitors have confirmed that North Korea has restored their access to the Yongbyon nuclear site, including the plutonium reprocessing plant.
The decision came a day after Pyongyang pledged to resume disabling the plant, and two days after the North was taken off a US list of terrorism sponsors.
Pyongyang had sought removal from the US blacklist since agreeing a nuclear disarmament-for-aid deal in 2007.
The secretive communist state tested an atomic weapon in 2006.
But six-nation talks between North and South Korea, the US, China, Japan and Russia later resulted in an agreement for Pyongyang to halt all its nuclear work.
The current dispute flared when the North barred nuclear inspectors after accusing the US of failing to honour its side of the deal by keeping the country on its terror blacklist.
In a statement, and IAEA spokeswoman said inspectors would start overseeing "core discharge activities" at the Yongbyon reactor on Tuesday.
She said the inspectors would also resume "containment and surveillance measures" at the site.
Reuters news agency quoted a diplomat as saying the first job of the inspectors would be to reassess the status of the nuclear facilities since recent steps taken to reactivate them.
Pyongyang began disabling its Yongbyon nuclear reactor in August, but recently made moves to reactivate the plant after the US initially stalled on removing North Korea from the terror blacklist.
On Saturday the US state department said North Korea had agreed to allow nuclear experts to take samples and conduct forensic tests at all its declared nuclear facilities and undeclared sites, prompting Washington to reverse its decision.
The North would also allow inspectors to verify its denials about transfers of nuclear technology and an alleged uranium programme, it said.
South Korea hailed the US decision to remove the North from its blacklist, saying it would help lead to its "eventual abandonment of its nuclear programmes".
But Japan criticised the move. Finance Minister Shoichi Nakagawa said Tokyo first wanted North Korea to provide more information about Japanese citizens it abducted in the 1970s and 1980s, in what he said amounted to terrorist acts.
North Korea has admitted to kidnapping 13 Japanese civilians to train its spies. Japan believes the figures could be higher and is sceptical about North Korea's claim that all but five are now dead.