North Korea's decision to close its Yongbyon reactor is positive but has no impact on South Korea's ratings as Pyongyang is unlikely to give up all its nuclear weapons, an international ratings agency said.

Standard and Poor's said the shutdown was encouraging but the peninsula's geopolitical risk levels and its own sovereign credit ratings for South Korea remained unchanged because the risk of the North's nuclear programme continued.

"If we assume the single most important policy objective of North Korea's regime is its own survival, it is difficult to imagine the leader(s) of North Korea would give up their nuclear capabilities entirely in exchange for economic benefits," S and P said in a statement.

"Furthermore, North Korea's behaviour is difficult to predict -- as its past record of failing to meet agreements demonstrates. In this sense, there is still a risk regarding the nuclear disarmament of North Korea."

UN nuclear inspectors confirmed Monday that the North has closed its Yongbyon reactor, the first step in a peace deal under which it would disable all nuclear programmes in exchange for aid and economic, diplomatic and security benefits.

The ratings agency said the United States and North Korea could differ on the meaning of "disable" -- the word used in a February 13 pact -- which was less clear than "dismantle."

It said the six-nation deal also also did not cover the treatment of all North Korea's existing nuclear capabilities or the enriched plutonium it is said to possess, which could produce around four to eight nuclear warheads.

"Even if the future enrichment of plutonium can be avoided, there remains the risk of nuclear brinkmanship on the part of North Korea," S and P said.

The agency said it could not fully discount possible war on the peninsula, although its baseline scenario remains a peaceful solution. "However, we believe that any such peace could be protracted and inconclusive."

It said there was also a possibility of Korean reunification, the cost of which "remains a contingent risk to South Korea'=s fiscal position and outstanding debt."

By comparison, Moody's Investors Service said earlier this month it had placed South Korea's ratings on review for a possible upgrade, partly because of progress towards scrapping North Korea's nuclear programme.