A team of American nuclear weapons experts, who were invited to visit North Korean nuclear weapons sites, reports that North Korea definitely has nuclear grade plutonium. But no one actually saw a nuclear bomb, and North Korea has not tested such a device. North Korea, which has been trading technology with Pakistan, could very well have one or more nuclear bombs. But unless they test their design, they cannot be sure that their bomb works. This was the case, in 1998,  when India and Pakistan each conducted several nuclear weapons tests. Some of Pakistan's bombs didn't work (the high explosive "trigger" went off, and simply spread dispersed the nuclear material without causing a nuclear explosion.) Even if North Korea has a successful nuclear bomb test, it must then make the weapon small and rugged enough to be delivered successfully by an aircraft or missile warhead. This is not easy, as older American and Russian nuclear weapons scientists and engineers have explained in books and articles.