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.