Washington-based 38 North said Saturday that images from March 3 and 17 suggested that an experimental light water reactor (ELWR) at the country's Yongbyon site was "nearing completion".

Construction had started on a new support building, while water discharges were detected from the reactor's cooling system, suggesting that the ELWR "is nearing a transition to operational status", the report said.

The images also indicated that a five-megawatt reactor at Yongbyon continued to operate and new construction had started around the site's uranium enrichment plant, likely to expand its capabilities.

"These developments seem to reflect Kim Jong Un's recent directive to increase the fissile material production to expand its nuclear weapons arsenal," the report added.

On Tuesday, North Korean state media unveiled photos of Kim inspecting new, smaller nuclear warheads as he ordered an expansion in "the production of weapon-grade nuclear materials" for an "exponential" increase in the North's arsenal.

Experts said the potential tactical nuclear weapons did indeed show key signs of technical progress, and could indicate an imminent nuclear test.

Officials in Seoul and Washington have warned since early 2022 that North Korea may conduct its seventh nuclear test.

Seoul and Washington have ramped up joint military drills as North Korea conducts ever more provocative weapons tests while blaming the South for the deteriorating security situation.

A commentary carried by the North's Korean Central News Agency (KCNA) on Sunday slammed the series of joint drills by the allies, warning of its "nuclear attack capability".

"It will be quite clear how the DPRK's nukes will be used if the war deterrence does not work on those who are running amuck, not being afraid of nuclear weapons," it said.

The North Korean military has carried out multiple drills in recent weeks, including the test-firing of what state media described as an underwater nuclear-capable drone and the second launch of an intercontinental ballistic missile this year.

Last year, North Korea declared itself an "irreversible" nuclear power, while in March, Kim ordered the North Korean military to intensify drills in preparation for a "real war".