North Korea said on Sunday it was not saying "empty words" when it came to its nuclear capabilities, adding that the United States and South Korea were engaging in "wrong behavior by putting themselves at grave risk," according to North Korean state media.

The North's official Korean Central News Agency (KCNA) has sharply criticized the two allies' ongoing joint military exercises and plans to hold massive live-fire exercises in June.

"This reminds the people and military of North Korea of June 1950 when they should have been the target of a war catastrophe, and increases their vigilance," she said.

"The United States and its followers should never forget the fact that their rival country has practically a nuclear offensive capability, as well as the characteristics of the North Korean people and military that say no empty words," she stressed.

Nuclear activity at the Yongbyon site

A U.S. research site reported that 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.

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

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 that reactor's cooling system has also been detected, and 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.