South Korea's president said he is worried that North Korea might backtrack on its pledge to dismantle its nuclear capability because the communist regime is "unpredictable."

North Korea agreed earlier this week to shutter its main nuclear complex within 60 days and then disable all its nuclear facilities in exchange for receiving the equivalent of up to 1 million tons of heavy fuel oil and political incentives from the United States and other regional powers.

It was the first time that North Korea agreed to take specific disarmament steps since six-nation talks began in 2003 among China, Japan, the two Koreas, Russia and the United States.

"What I'm most concerned about is North Korea. It's true that North Koreans are a bit unpredictable," South Korean President Roh Moo-hyun told a meeting of South Korean expatriates in Rome on Thursday, according to a transcript of his comments provided by his office in Seoul.

North Korea's main nuclear envoy, Kim Kye Gwan, said Thursday his nation would honor the deal, according to Japan's Kyodo news agency.

But on Friday, Pyongyang gave no word on the landmark deal, as it celebrated leader Kim Jong Il's birthday, one of the country's biggest national holidays.

North Korea has long been known for its unpredictability and has a track record of violating agreements. In September, 2005, Pyongyang agreed in principle to disarm in exchange for aid and security guarantees, but it backtracked on that pledge a day later.

Still, Roh said he was "optimistic" about the accord's implementation and that South Korea would try to make sure the deal is carried out by taking advantage of Seoul's economic "leverage" over Pyongyang.

Roh also said he felt this week's deal is so crucial that the South would have paid for the whole thing.

Four of the five countries in continuing disarmament talks — China, South Korea, Russia and the United States — have agreed to share the aid costs for the impoverished North. Japan refused unless the issue of its citizens abducted by North Korea is resolved.

"I thought that even if we gave them all, even if we bear it all, we should resolve this problem," Roh said.

The comments signal that South Korea, which has been holding off on aid shipments pending progress at the nuclear talks, could soon resume major assistance to the North.

The two Koreas agreed this week to reopen their highest-level regular dialogue channel between Cabinet-level officials of the two sides, which have been suspended since July over the North's missile and nuclear tests.

Aid provision is expected to be a key topic for the Cabinet-level meeting scheduled for later this month.

South Korea's nuclear envoy, Chun Yung-woo, said he believes the North would honor this week's deal because it offers not only economic aid, but also political incentives from the U.S. that the North has long demanded, including bilateral normalization talks, its removal from Washington's list of states sponsoring terrorism and the lifting of U.S. financial restrictions.

"Where else would North Korea get these unless it does not keep its denuclearization commitments?" Chun said.

Late Thursday, Chinese President Hu Jintao spoke by phone with U.S. President George W. Bush, reaffirming Beijing's commitment to help implement the landmark deal with the North, according to China's official Xinhua News Agency.