Satellite images showed a high level of activity at North Korea's main nuclear site, after the country's leader ordered increased bomb fuel production to expand the country's nuclear arsenal, a US research site reported.

The Washington-based North 38 North website said in a report that the activity, which it monitored according to images taken on March 3 and 17, could indicate that a pilot light-water reactor at the Yongbyon site is nearing completion and moving into operation.

The report said images showed that a 5-megawatt reactor at Yongbyon continued to operate, and that construction had begun on a support building around the experimental reactor.

Water discharge from the reactor's cooling system was also detected. New construction has begun around the Yongbyon uranium enrichment plant, likely to expand its capacity.

"These developments appear to reflect the latest directive by (North Korean leader) Kim Jong Un to increase the country's production of fissile material to expand its nuclear weapons arsenal," the report said.

Pyongyang on Tuesday unveiled new, smaller nuclear warheads and pledged to produce more weapons-grade nuclear material to expand its arsenal, while denouncing intensified joint military exercises between South Korea and the United States.

It remains unclear whether North Korea has fully developed the small nuclear warheads needed to fit the smaller weapons it has displayed.

Reuters quoted analysts as saying mastering the production of such warheads would likely be a key goal if the country resumed nuclear testing for the first time since 2017.

South Korea and the United States have been warning since early 2022 that North Korea could resume nuclear testing at any time.