North Korea has enough weapons-grade plutonium stockpiled over the past five years to build about four to eight crude nuclear warheads, a US think-tank says.
Washington-based Institute for Science and International Security (Isis) said on Tuesday that North Korea currently has between 28kg and 50kg of weapons-grade plutonium.
North Korea signed a six-nation nuclear pact in Beijing last week requiring it to shut down its main nuclear reactor by mid-April in return for fuel aid.
The pact, however, is silent about existing plutonium stocks of between 46 and 64kg or the nuclear weapons the North has already built.
The institute said in its report that North Korea may have obtained designs for a warhead from the nuclear black market run by Pakistan's AQ Khan.
Such a bomb could possibly be mounted on North Korea's medium-range Nodong missile, which has an estimated range of about 1,000km, it said.
Most of the weapons-grade plutonium, it added, was produced since late 2002 after the US accused the North of running a secret uranium enrichment programme.
North Korea's alleged secret programme was not addressed in the report which Isis claims is based on scientific estimates, publicly available information and visits to facilities by its experts.
No confirmation was available about the North's nuclear arsenal given its refusal to publicly disclose such information.
The institute also speculated about North Korea's possible nuclear strategy in the event of a crisis saying it would test-fire a first warning before detonating a warhead over the ocean or on a ship.
If war broke out, North Korea would use nuclear weapons against South Korea or Japan, and possibly keep bombs in the country to be detonated when enemy forces arrive, the report said.