North Korea’s nuclear arsenal received an additional four to six weapons in the last 18 months, a U.S. think tank said in a report released Tuesday. South Korea’s neighbor has been testing nuclear weapons and missiles since January, much to the anger of its neighbor Seoul and its ally Washington.

The Institute for Science and International Security’s report said that with the latest addition, Pyongyang’s current nuclear armory stands at 13 to 21 weapons. The total estimate did not take into account the likely production of additional highly enriched uranium at a second centrifuge plant believed to exist in the North.

North Korea North Korean leader Kim Jong Un guides on the spot the underwater test-fire of strategic submarine ballistic missile in this undated photo released by North Korea's Korean Central News Agency (KCNA) in Pyongyang on April 24, 2016. Photo: REUTERS/KCNA

“As of June 2016, North Korea has about 13-21 nuclear weapons, where one weapon was subtracted to reflect the underground test in early 2016 … Nonetheless, this estimate, despite not being comprehensive, shows that North Korea appears to be significantly increasing its nuclear weapons capabilities,” the report said.

The assessment came after North Korea reportedly started its Yongbyon plutonium plant, which was shut down in July 2007. Government officials in Pyongyang had said in September 2015 that the plant was operational again. They had said that the plant was working on improving the “quality and quantity” of its nuclear weapons.

While, the amount of weapons-grade uranium and plutonium that the reclusive state owns is unclear, reports say that the plutonium manufactured at Yongbyon is used in the North’s nuclear bombs.

The Kim Jong Un-led regime has conducted a series of missile and nuclear tests since the beginning of this year, and the leader has called for the strengthening of the country’s defense capabilities. Pyongyang also claims to have secured intercontinental ballistic missile technologies. The country faces strict sanctions from the United Nations Security Council over its fourth nuclear test in January and a rocket launch in February.