Satellite images have revealed a high level of activity at North Korea 's main nuclear complex , a think tank said, after the country's leader Kim Jong Un called Tuesday for increased production of "military nuclear material".

The US-based 38 North said that satellite imagery, taken between March 3 and 17, indicated that construction of a light water reactor at the Yongbyon site was "almost completed".

The report of the intellectual organization stated that the images reveal that the construction of a building has begun and water discharge from the cooling system has been monitored, indicating that the reactor is preparing for its "operation".

He added that a 5-megawatt reactor in Yongbyon continues to operate and that new construction work has begun around the site's uranium enrichment plant, likely to increase its capacity.

"38 North" said, "These developments seem to reflect Kim Jong-un's recent directive to increase the production of fissile material to bolster his nuclear arsenal."

On Tuesday, North Korean state media published pictures of Kim inspecting a row of small warheads that appear to be tactical nuclear warheads, and he called for an increase in the production of "military nuclear materials" in order to "significantly" increase the production of nuclear weapons.

Experts believe that tactical nuclear weapons are an indication that North Korea is making technical progress and appears to indicate that the country is preparing for a seventh nuclear test.

Seoul and Washington have been warning North Korea since the beginning of 2022 of a new nuclear test.

The two allies have stepped up their joint military exercises, while Pyongyang continues to conduct missile tests and accuses Seoul of causing the deterioration of regional security.

On Sunday, the official Korean Central News Agency condemned the series of joint exercises conducted by Washington and Seoul, warning of North Korea's "nuclear attack capability".

The North Korean military, which has doubled its military exercises in recent weeks, said Tuesday it had conducted a second test of what it described as an "unmarried underwater nuclear attack" mechanism.

Pyongyang confirmed in 2022 that its status as a nuclear power is "irreversible", and the North Korean leader ordered his army this month to intensify military exercises for a "real war".