A Japanese daily says the United States believes North Korea has used around 18 kilograms of plutonium for its nuclear program.

The Tokyo Shimbun also said Monday that Pyongyang used six kilograms of plutonium for its underground nuclear test in October 2006.

The paper cited remarks made by North Korea's top nuclear envoy Kim Kye-kwan during a meeting last December. Kim is said to have told his U.S. counterpart Christopher Hill that his country had produced about 30 kilograms of plutonium.

However, the daily said Washington estimates that Pyongyang produced around 50 kilograms of the radioactive substance that can be used to create nuclear weapons.