SEOUL, South Korea — South Korea said Wednesday that North Korea has started work to restore its nuclear facilities after the communist country suspended operations last month to disable them.

The Foreign Ministry said South Korea, the United States and other members of a six-party group involved in nuclear negotiations with the North are working together closely to determine how to respond to the North's latest move.

North Korea said last week it had stopped disabling its nuclear reactor on Aug. 14 and threatened to restore the Yongbyon plutonium-producing facility. It accused Washington of failing to hold up its end of the disarmament deal because it had not yet removed the North from its list of state sponsors of terrorism.

The U.S. responded by repeating its demand that North Korea must first agree to a plan to verify an accounting of nuclear programs it submitted in June, if it wants to be taken off the list.

White House press secretary Dana Perino said she had heard the reports, but could not confirm that North Korea had begun to reassemble its nuclear facility.

"We will be taking an assessment along with our international partners. This is not a decision that the United States makes alone," she told reporters aboard Air Force One as it flew to Louisiana where President Bush was surveying damage from Hurricane Gustav.

"We're obviously troubled by it and it's very unfortunate for the North Korean people who could be greatly helped by the action-for-action mechanism," she said. "And once North Korea simply agrees to a a verification protocol, then the United States will take them off of our state sponsor of terrorism list , but we're not going to do it without it."

North Korea carried out an underground nuclear test blast in October 2006.

But the country began disabling its nuclear plant in Yongbyon, north of the capital Pyongyang, in November. It later slowed the work to protest a delay in the provision of promised aid from its negotiating partners.

Disarmament efforts reported major progress in June after the North submitted its long-delayed nuclear declaration and destroyed its nuclear cooling tower in a show of its commitment to denuclearization.

The U.S. then announced it would delist the North from the terrorism blacklist, a coveted goal of the North's cash-strapped regime. But it says it has not done so yet because of the dispute over verification.

South Korea's government is "seriously concerned" over the North Korean move, a ministry statement said. It said South Korea was urging North Korea not to further aggravate the situation.

The Foreign Ministry refused to disclose how it confirmed the North had began work to restore its nuclear facilities.

The South Korean statement was vague about the exact timing of the North's resumption of work on its nuclear facilities. South Korea's Yonhap news agency reported that the work began Wednesday.

South Korean and U.S. officials have said that it would take at least a year for the North to restart the facilities after they are completely disabled.

The Vienna-based International Atomic Energy Agency said it had no immediate comment on Wednesday's claims.