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.