Hyun Hak Bong said the US was making unreasonable demands
North Korea has stopped disabling the Yongbyon nuclear reactor and is making "thorough preparations" to restart it, a foreign ministry official has said.
Hyun Hak-bong said that Pyongyang had suspended work to put the plant out of action because the US had not fulfilled its part of a disarmament-for-aid deal.
When asked when it would be restored, he said: "You'll come to know soon."
Last month, North Korea said Washington had not removed it as promised from a list of state sponsors of terrorism.
Pyongyang was expecting to be removed after finally submitting a long-delayed account of its nuclear facilities to the six-party talks in June, in accordance with the disarmament deal it signed in 2007.
It also blew up the main cooling tower of the Yongbyon facility in a symbolic gesture of its commitment to the process.
For its part, the US says it will not remove North Korea from its list of state sponsors of terrorism until procedures by which the North's disarmament will be verified are established.
Mr Hyun claimed the process of decommissioning the plutonium-producing reactor at the Yongbyon plant was 90% complete.
But he said Pyongyang would respond to the US by halting the process and "proceeding with works to restore [the reactor] to its original status".
"You may say we have already started work to restore them," he told reporters in the border village of Panmunjom inside the Demilitarised Zone before sitting down for talks with South Korean officials on sending energy aid to the North.
US State Department spokesman Sean McCormack told reporters that although the reactor was not yet operational, "we would urge them not to get to that point".
Some South Korean officials have expressed doubt over whether the North's claims to be reconstructing Yongbyon are genuine or a ploy to exert pressure on Washington.
'Year to restore'
Mr Hyun also warned the US not to push for inspectors to verify the disarmament process work already undertaken, saying it was never part of the six-party deal.
"The US is insisting that we accept unilateral demands that had not been agreed upon. They want to go anywhere at any time to collect samples and carry out examinations with measuring equipment," he said. "That means they intend to force an inspection."
The diplomat said compelling Pyongyang to permit a "robber-like inspection method in the name of an international standard" would exacerbate tensions.
Meanwhile, South Korea said it would fulfil the deal struck at six-party talks to supply the North with a million tons of heavy fuel oil or equivalent in exchange for nuclear disablement.
Nearly half has been delivered and AFP news agency quoted a South Korean negotiator, Hwang Joon-kook, as saying the rest would be sent.
"We also want to make sure that the six-party process does not go backward," Mr Hwang was quoted as saying.
In separate remarks by North Korea's Mr Hyun, he reiterated North Korea's rejection of reports that leader Kim Jong-il remains in ill health after suffering a stroke. He called the reports "sophism by evil people".
Earlier this month, reports in Japan, backed up by South Korea's foreign ministry, claimed the North Koreans were actively reconstructing Yongbyon.
The US later said plant workers appeared to be moving equipment out of storage, but that there was no effort to reconstruct it.
Experts believe Yongbyon would take a year to restore, a view supported by a recent International Atomic Energy Agency report.
The IAEA said the regime had already removed large quantities of essential nuclear materials from Yongbyon even before it agreed to dismantle the plant.