North Korea still poses a military threat even if the reclusive state has started to roll back its nuclear arms program, South Korea's defense minister said on Wednesday.

On Monday, North Korea started to decommission its nuclear complex that produces arms-grade plutonium as part of a disarmament-for-aid deal it reached with regional powers including the United States and South Korea.

Although it is true that North Korea has begun the process of disabling its nuclear program, we cannot say that the threat from North Korea has reduced tangibly or discernibly, Kim Jang-soo said after meeting U.S. Defense Secretary Robert Gates.

Kim and Gates discussed how the allies could counter the threat posed by the North's 1.2 million-man military, most of whom are near the heavily fortified border with the South.

The North Korean nuclear and conventional threat remains the focal point of our alliance for its deterrent and defense position, Gates said at a joint news conference.

The United States has about 28,000 troops in South Korea to support the country's military of some 670,000 soldiers. The two Koreas remain technically at war because their 1950-1953 conflict ended in a truce and not a peace treaty.

A U.S. State Department official who just returned from the North's Yongbyon nuclear complex said the state has started to reverse operations at three key facilities -- an ageing reactor, a plant that produces nuclear fuel and another that turns spent fuel into arms-grade plutonium.

The deal requires North Korea to disable the three by the end of 2007, provide a list of its nuclear arms activity, account for all its fissile material and answer U.S. suspicions that it has a clandestine program to enrich uranium for weapons.

Under the deal it reached with China, Japan, Russia, South Korea and the United States, the energy-starved North is to receive 1 million tonnes of heavy fuel oil or equivalent aid.

The United States will also move toward taking North Korea off a U.S. terrorism blacklist.

Proliferation experts said the disablement steps would be reversible but would put the North out of the plutonium production business for about a year.

U.S. officials estimate the North has about 110 lb of plutonium. Proliferation experts say that is enough for six to eight bombs.