War of words is pushing up the cost of insuring South Korean debt. North Korea's test of the Pukguksong-2 missile in the undated photo released by the Korean Central News Agency (KCNA) in Pyongyang February 13, 2017. Photo: Reuters/KCNA

North Korea has enough fissile material for about 10-20 nuclear warheads, according to the Stockholm International Peace Research Institute (SIPRI).

UPI, quoting a report issued by the Swedish think tank on Monday, said the latest estimate of 10-20 warheads represents an increase from previous years. Prior estimates, from various international sources, had speculated that North Korea had less than 10 such devices in its arsenal.

The report added that Pyongyang had “carried out an unprecedented number of flight tests of different missile systems in 2016 with mixed results.”

But SIPRI says it’s unlikely that North Korea has developed a sufficiently compact nuclear warhead capable of reaching the continental United States.