North Korea has enough plutonium to build an estimated four to eight crude nuclear warheads that could potentially be mounted on medium-range missiles, a Washington-based think tank said.

The amount of radioactive material in the communist nation is key to monitoring its compliance with a Feb. 13 agreement to disarm, under which the North is required to declare all nuclear programs and material to international inspectors.

The Institute for Science and International Security said in a report released Tuesday that as of this month North Korea had between 101 and 141 pounds of plutonium, of which between 62 and 110 pounds is estimated to be usable for weapons — enough to make four to eight crude warheads.

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It said most of the separated plutonium for bombs — possibly as much as 99 percent — was produced since late 2002, when the latest nuclear crisis with North Korea began after the U.S. accused it of running a secret uranium enrichment program.

The alleged uranium program wasn't addressed in the report, which the ISIS said was based on scientific estimates and other publicly available information, along with visits to the actual facilities by ISIS experts.

The report said North Korea had likely obtained designs for a nuclear warhead from the nuclear black market run by Pakistan's A.Q. Khan. Such a bomb could possibly be mounted on North Korea's Nodong missile, which has an estimated range of about 620 miles, it said.

No confirmed information is available about the North's arsenal given its refusal to disclose such information publicly.

The ISIS report speculated about North Korea's possible nuclear strategy in the event of a crisis, saying it would first likely conduct another nuclear test in an attempt to head off a further escalation. After that, the North could detonate a warhead over the ocean or explode one on a ship as a further warning, it said.

If war broke out, North Korea would be expected to use nuclear weapons against South Korea or Japan and possibly even keep bombs inside the country to be detonated when enemy forces arrive, the institute said.

After more than three years of talks, North Korea agreed earlier this month with the U.S. and other regional powers to take its first steps toward disarming since the latest nuclear standoff began. Under the pact it is required to shut down its main nuclear reactor by mid-April and eventually would receive the equivalent of 1 million tons of heavy fuel oil for disarming.

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