The U.S.’ Institute for Science and International Security(ISIS) has assessed that North Korea has around 45 nuclear weapons.

The nonprofit institute made the assessment in a report released on Monday titled “North Korean Nuclear Weapons Arsenal: New Estimates of its Size and Configuration.”

The report calculated an average rounded estimate of 45 nuclear weapons, with a range of 35 to 65, that would include a mix of composite core nuclear weapons, thermonuclear weapons, and plutonium-only and weapon-grade-only nuclear weapons.

The institute said such an estimate is based on the North’s production of plutonium and weapons-grade uranium.

The report said the production of such key materials can be estimated with commercial satellite observations, previous North Korean declarations, reports released by the International Atomic Energy Agency and the reporting of various intelligence agencies, among others.

The report noted, however, that “due to a lack of information about North Korea’s nuclear weapons production complex, all of these estimates have relatively large uncertainties.”

ISIS President David Albright, who wrote the report, told Radio Free Asia that he believes the North is capable of producing four to 12 nuclear weapons in a year.

The report also said that the “Hwasan-31,” which the regime claims is a tactical nuclear warhead, has an outer casing with “a diameter of about 40 to 45 centimeters and could be a levitated fission weapon with a predicted yield of about ten kilotons.”