North Korea’s nuclear arsenal has grown “significantly” to a total of 13 to 21 weapons, with the communist country procuring an additional 4 to 6 nuclear weapons in the last 18 months, a U.S. think tank said in a report Tuesday.

The analysis by the Institute for Science and International Security comes days after media reports indicating that the hermit kingdom has resumed reprocessing nuclear fuel to harvest plutonium. The South Korean government had said that it is keeping a close eye on Pyongyang with “great concern” upon the allegations.

In its report, the institute said that the analysis on the production of separated plutonium, weapon-grade uranium and nuclear weapons put the estimated number of North Korean nuclear weapons at 10-16 at the end of 2014. 

Based on the additional production of nuclear materials at the North’s main Yongbyon nuclear facility, the institute estimated the addition of 4 to 6 nuclear weapons since then. This brings the total number to its aforementioned estimate, as one weapon has been detonated during Pyongyang’s Jan. 6 nuclear testing.

The report noted that the upper bound may actually be greater, since the initial estimate disregarded any contribution from the second centrifuge plant in the facility.

South Korean military estimates the North to have around 40 kilograms of plutonium, while the amount of highly-enriched uranium remains unclear.

Pyongyang’s leader Kim Jong-un has been demanding his troops to step up efforts for the country’s nuclear and ballistic missile programs, which is in violation of the U.N. Security Council resolution. The UNSC in March passed a new resolution that included the strongest sanctions against the reclusive country to date.  

North Korea officially proclaimed the status of a nuclear-armed state during its ruling party congress last month.

By Yoon Min-sik(minsikyoon@heraldcorp.com)