(CNN) -- North Korea is preparing to restart a reactor at the key Yongbyon nuclear complex, the South Korean news agency reported Thursday.

Hyun Hak-Bong, a chief North Korean negotiator at six-nation talks, told reporters his country was "thoroughly preparing to restart" the reactor and that reporters would "know soon" when his country would do that, the Yonhap news agency said.

The North has agreed to abandon its atomic weapons program on a promise that it would receive energy aid equivalent to one million tons of heavy fuel oil from nations that have participated with the North in six-party talks -- China, Japan, South Korea, Russia and the United States.

North Korea has also agreed to disable the Yongbyon nuclear complex by October in exchange for a pledge from the United States to lift some sanctions and remove North Korea from a list of countries that sponsor terrorism.

In June, North Korea declared in a 60-page document that it had produced enough plutonium since 1986 for seven nuclear bombs. The North publicly destroyed a water-cooling tower at the Yongbyon complex soon after its declaration.

Progress has stalled since then.

North Korea has said it stopped disabling the Yongbyon complex on August 14. It also said it would consider rebuilding the reactor because the United States had not removed it from a list of countries that sponsor terrorism.

A U.S. State Department spokesman, Robert Wood, said then that the North would be taking "a step backward" if it rebuilt the reactor.

On September 3, North Korea started to reassemble parts of the complex, the Yonhap news agency reported, citing diplomatic sources.

A spokesman for the U.S. National Security Council, Gordon Johndroe, said then that "the process is not moving in the direction we want to move in.

"We think North Korea is taking these steps because it has not been removed from the terrorism list," he said.

One sticking point between the United States and North Korea involves verification.

Washington has said it will not remove North Korea from the terrorism list until North Korea agrees to set up an internationally recognizable mechanism to verify its declaration.

The United States has demanded that inspectors be given the right to visit all suspected nuclear facilities without notice, the South Korean news agency said.

North Korea rejects that provision.

"The U.S. is gravely mistaken if it thinks it can make a house search in [North Korea] as it pleases just as it did in Iraq," the North Korean Foreign Ministry said in a statement carried by the North's state-run Korean Central News Agency.