North Korean leader Kim Jong Un visits the Korean People's Army's Factory in this undated photo released by North Korea's Korean Central News Agency on November 15, 2014. (Kcna/Reuters)

With its trademark belligerence, North Korea threatened Thursday to conduct another nuclear test in response to the “grave political provocation” of international efforts to charge the country’s leader with crimes against humanity.

The threat came as a U.S. think tank said North Korea appeared to be restarting its Yongbyon nuclear plant, possibly to process weapons-grade plutonium.

The developments mark a return, at least temporarily, to bluster after a months-long charm offensive in which Pyongyang tried to thwart a U.N. resolution condemning the North Korean leadership for decades of human rights abuses.

A U.N. General Assembly committee on Tuesday approved the resolution, sponsored by the European Union and Japan. It paves the way for Kim Jong Un and his cadres to be referred to the International Criminal Court.

“America’s aggression toward North Korea is making us unable to refrain any longer from a new nuclear test,” the Foreign Ministry in Pyongyang said in a statement published Thursday, adding that the North’s “war deterrence capabilities” would be strengthened “to guard against forceful American invasion plots.”

North Korea has conducted three nuclear tests since its first in 2006, the most recent being in February last year. But it refrained from a fourth test and instead embarked on an unprecedented flurry of diplomacy after a U.N. inquiry commission released a groundbreaking, 372-page report early this year that detailed human rights abuses including brainwashing, torture, deliberate starvation and executions in the country.

North Korea appeared genuinely worried that Kim would be held to account, analysts said, and that prompted it to engage on human rights issues in ways it never has before.

In a sudden move, North Korea recently released three Americans it had detained. It also tried to cut a deal with a U.N. special rapporteur that would grant him access to the country in exchange for the ICC referral being dropped.

On Thursday, there was a change in tone. “We fully reject the forceful passing of this draft resolution led by the U.S. as it tries to overthrow this socialist state that is built on its people,” the Foreign Ministry statement said.

The nonbinding U.N. committee resolution will be put on the Security Council’s agenda. The focus will then shift toward efforts to stop China and Russia — North Korea’s main allies, which have often sided with it in Security Council votes — from using their vetoes.

China, which voted against the resolution in committee, said Wednesday that the Security Council was “not an appropriate place” to discuss human rights and described sending North Korea to the ICC as “not helpful.”

Separately, the 38 North Web site, part of the U.S.-Korea Institute at Johns Hopkins University’s School of Advanced International Studies, reported suspicious activity at the Yongbyon plutonium production reactor in North Korea, including steam emanating from a cooling tower.

The reactor had been shut down for more than 10 weeks, longer than would be needed for routine maintenance, analyst Nick Hansen wrote.

He said it was too soon to reach a firm conclusion but warned of indications that North Korea could be preparing to restart a laboratory where weapons-grade plutonium is produced, using spent nuclear fuel rods.