"
The IAEA has not shown any positive attitude
"
North Korean news agency
The Yongbyon reactor was closed down in 1994 as part of an American-led deal to try to halt North Korea's alleged nuclear programme.
Earlier this month, Pyongyang said it would start using the reactor again and demanded that the International Atomic Energy Agency (IAEA) - the UN's nuclear watchdog - remove seals and surveillance cameras from the site.
North Korea said it needed to increase its electricity production, after the United States decided to suspend oil shipments to the country.
Regional fears
The IAEA voiced deep regret at Pyongyang's decision to reactivate its nuclear programme - which correspondents said effectively marked the end of the so-called "Agreed Framework" of 1994.
But the Pyongyang government, said the agency was responsible for the move.
"The IAEA has not shown any positive attitude, whiling away time after proposing what it called working negotiations," North Korea's official Korean Central New Agency said.
"The situation compelled the DPRK (North Korea) to immediately start the work of removing the seals and monitoring cameras from the frozen nuclear facilities for their normal operation to produce electricity."
The US and its regional allies - South Korea and Japan - are worried that the plant could also be used as part of a wider nuclear weapons programme, which North Korea has said it is entitled to.
Analysts say the Yongbyon reactor could produce plutonium needed for a nuclear device.
The US decided to suspend oil aid to Pyongyang in October, following North Korea's alleged admission that it was actively pursuing a nuclear weapons option.
In a speech in January US President George W Bush mentioned North Korea as part of the "axis of evil" of countries illegally developing weapons of mass destruction.
The US believes the reactor and extraction plant have been used to produce plutonium - possibly enough for one or two nuclear weapons. Activities at site frozen under 1994 Agreed Framework
Taechon: 200-MWe nuclear power reactor - construction halted under Agreed Framework
Pyongyang: Laboratory-scale "hot cells" that may have been used to extract small quantities of plutonium
Kumho: Site of two 1,000-MWe light water reactors under construction by Kedo