North Korea is estimated to have produced enough weapon-grade plutonium for about two to four more nuclear weapons than previously assessed, the US-based Institute for Science and International Society said Monday.

The institute based the speculation on the International Atomic Energy Agency’s recent evaluation of the communist country’s nuclear capacity.

In the Aug.19 report, the IAEA said Pyongyang appears to have unloaded fuel from its 5-megawatt electric reactor at its main Yongbyon nuclear facility in late 2015 and processed it to separate plutonium in the radiochemical laboratory in the first quarter of 2016.

“In this case, we have estimated that North Korea could have produced and separated an additional 5.5-8 kilograms of weapon-grade plutonium,” David Albright of the ISIS said in a report.

“This amount of plutonium is enough for two to four nuclear weapons, assuming 2 to 4 kilograms of weapon-grade plutonium per weapon, or a central estimate of three additional nuclear weapons based on plutonium.”

The IAEA reported that its monitoring of the Yongbyon site -- which include satellite imagery -- indicated comprehensive activities at nuclear-related facilities.

It said indications of the reactor’s operation had ceased to appear between mid-October and early December last year, during which the reactor may have been defueled and refueled.

But the organization added that such observations cannot be confirmed, as it does not have access to Yongbyon or the uranium concentration plant and uranium mine in Pyongsan.

ISIS estimates that the communist North currently has about 13 to 21 nuclear weapons as of this summer, including the weapon-grade uranium as well. The tally excludes the addition of weapon-grade uranium production during the past 18 months that may have occurred at a second enrichment plant outside Yongbyon.

“If this second plant was included, the upper bound of 21 would increase by a few nuclear weapons,” Albright noted.

The think tank in June said North Korea appears to have procured an additional four to six nuclear weapons in the past 18 months.

South Korean experts have presumed that the North has approximately 40 kilograms of weapon-grade plutonium.

North Korean leader Kim Jong-un reaffirmed his nuclear ambition during the ruling Workers’ Party of Korea’s Congress in May, followed by the country’s first successful intermediate Musudan ballistic missile launch in June.

On Monday, the hermit kingdom blasted the annual South Korea-US joint military drill Ulchi Freedom Guardian as “a phased scheme for invasion,” and threatened a pre-emptive nuclear strike against the allies.

It has claimed that the joint drills, such as the UFG and the annual Key Resolve exercise held earlier in the year, drives up tensions in the Korean Peninsula.

But a new study by the US-based Center for Strategic and International Studies denied correlation between the joint exercises and the North Korean provocations.

Beyond Parallel, a CSIS-affiliated website, analyzed the North Korean provocations from 2005 to 2016 and found that the said exercises have had “null effect.”

Data showed instead that the state of Washington-Pyongyang diplomatic relations prior to the exercise better indicated whether there would be provocative actions by the North -- with worse bilateral relations leading to a higher chance of belligerence during and after the drills.

“The Beyond Parallel dataset really undercuts the argument that the US and ROK are the ‘bad guys’ on the peninsula riling tensions with twice-yearly military exercises. ... It’s just untrue,” Victor Cha, CSIS senior advisor and Korean chair, was quoted as saying.

The heighten tension is attributed to the leadership change in Pyongyang, the report found, as both the intensity and frequency of the country’s belligerence stepped up after former leader Kim Jong-ill -- Kim Jong-un’s father -- was confirmed to have suffered a stroke.

Before he suffered the stroke, there was an average of 2.5 provocative acts per year from North Korea, but this has now jumped to 8.9 per year. This includes ballistic missile launches and nuclear tests, according to the report.

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