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 US dropped on Hiroshima during World War II; a minority of US analysts believe the communist country may already have made more.
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.
A CIA spokesman declined last week to expand on the agency's conclusions.
North Korea has suggested it may conduct a nuclear test to demonstrate it is a nuclear power. But US officials are not sure that the North Koreans would expend a nuclear weapon if they have only a few.
"A North Korean decision to conduct a nuclear test would entail risks for Pyongyang of precipitating an international backlash and further isolation," the CIA says. "Pyongyang at this point appears to view ambiguity regarding its nuclear capabilities as providing a tactical advantage."
The CIA's conclusion was reported in an unclassified letter to the Senate Intelligence Committee in August. That letter, along with similar communications from the Defense Intelligence Agency, the FBI and State Department, was obtained by the Federation of American Scientists, a watchdog group that focuses on security and intelligence matters.
North Korea's nuclear program, which the US demands an end to, has been the focus of intense diplomatic activity in the region.
North Korea frequently issues threats but has also taken part in six-country talks regarding its programs. US officials believe North Korea, long in a dire economic state, regards nuclear weapons as a way to exact aid and concessions from the rest of the world.
US intelligence officials have acknowledged uncertainties about North Korea's weapons programs. The Defense Intelligence Agency, in its letter to the Senate committee, said a once-feared North Korean missile, the Taepo Dong 1, now appears to be only a research and development platform that is not intended for operational use.
North Korea remains ready, however, to test the Taepo Dong 2 -- a newer, long-range missile that may be capable of reaching the US, the DIA says.
The defense agency vaguely suggests that such a test could take place either from North Korean soil or "perhaps in another country" that the agency did not name, although Iran and North Korea are known to have cooperated on missile projects in the past.
In their political analyses, the American intelligence agencies said the government of Kim Jong-il appears unlikely to crumble from within, although they differed on who would succeed Kim if he died.
"We lack reliable insights into the internal dynamics of his regime, however successor[s] to Kim would most likely come from the military," the DIA said.