A top South Korean military officer said Wednesday that he believes North Korea is trying to develop a nuclear warhead that is small enough to be carried by its missiles.

North Korea is believed to maintain enough plutonium to produce about half a dozen bombs, but it is not believed to have mastered the technology needed fit a nuclear weapon inside a missile warhead.

Gen. Kim Tae-young, chairman of South Korea's Joint Chiefs of Staff, told a parliamentary committee that he believes "North Korea has been pushing to develop a small warhead to be mounted on a missile," according to the general's office.

Kim said it was not clear whether the North had already manufactured such a warhead.

North Korea conducted an underground nuclear test in 2006, and its long-range missiles are believed to able to reach as far as the west coast of the United States.

Kim's remarks came at a time of increased tension on the Korean peninsula.

North Korea began disabling its main nuclear complex north of Pyongyang last November as part of an aid-for-disarmament pact with the U.S., South Korea, China, Russia and Japan. The North, however, stopped the disablement work and began reassembling the facilities in mid-August in protest at Washington's refusal to remove it from a blacklist of state sponsors of terrorism.

The U.S. pledged to remove the North from the blacklist after the regime submitted a long-delayed account of its nuclear programs in June. The U.S. later insisted the North would only be taken off the list after it agreed to an international inspection of its nuclear declaration.

Washington's top nuclear envoy visited Pyongyang last week to resolve the impasse, but it was unclear whether it produced any breakthrough.