Washington ? The CIA has concluded that North Korea has been able to validate its nuclear weapons designs without a nuclear test, the agency disclosed to Congress.

The intelligence service believes that conventional explosives tests, conducted since the 1980s, have allowed the North Koreans to verify their nuclear designs would work. The agency believes North Korea has one or two nuclear weapons similar to what the United States dropped on Hiroshima during World War II.

CIA officials do not describe the precise mechanism by which the North Koreans could have verified their designs. The explanation to Congress provides the rationale behind the agency’s conclusion that North Korea already has a nuclear weapon.

The relatively simple fission weapons that North Korea is believed to have produced would presumably detonate a precisely built shell of conventional high explosives around a plutonium core, and the tests may have involved the designs of that shell.

North Korea has suggested it may conduct a nuclear test to demonstrate it is a nuclear power. But U.S. officials are not sure that the North Koreans would expend a nuclear weapon if they have only a few.