SEOUL, South Korea  — North Korea said it may conduct a “new form” of nuclear test if the U.S. challenges its efforts to enhance deterrence through military drills.

Kim Jong Un’s government “is fully ready for next-stage steps which the enemy can hardly imagine in case the U.S. considers them as a ‘provocation,’” the foreign ministry said today in a statement. “It would not rule out a new form of nuclear test for bolstering up its nuclear deterrence.”

Released through the official Korean Central News Agency, the statement didn’t say exactly what kind of nuclear test it was considering or when its drills might begin. North Korea has conducted three underground atomic tests since 2006.

The warning marks the clearest signal in months the North remains open to conducting its fourth test after detonating an atomic device in February last year. The U.S. maintains it won’t rejoin six-party nuclear talks unless North Korea shows clear signs that it is rolling back its atomic ambitions.

“The North probably won’t immediately conduct a nuclear test,” Koh Yu Hwan, a professor of North Korean studies at Dongguk University in Seoul, said by phone. “It is putting pressure on the U.S. not to further delay negotiations or expand sanctions.”

North Korea has fired off at least 86 rockets since Feb. 27, including ballistic missiles banned under United Nations resolutions. The ministry denounced the UN Security Council for condemning the launches, defending them as part of drills to respond to annual U.S.-South Korean military exercises.

North Korea threatened nuclear missile strikes against the U.S. and South Korea while the allies conducted their joint exercises last year. President Barack Obama said in April the North didn’t have the ability to tip its long-range missiles with nuclear warheads.

North Korea’s warning came even as South Korean President Park Geun Hye proposed building closer links with the North to spur reunification in a speech on March 28, and last month the two nations held the first reunions in more than three years of families separated by the Korean War. The North rejected the South’s offer earlier this month to make family reunions regular.

North Korea confirmed March 27 that it had restarted its main nuclear reactor. In August, the Washington-based Institute for Science and International Security said that the North might have doubled the size of its uranium enrichment facility at Yongbyon, citing satellite images.

Highly enriched uranium provides a second track to developing nuclear arms in addition to plutonium. Pyongyang has said it’s enriching uranium at a low level to produce energy.

“North Korea is implying in the foreign ministry statement it is seeing progress in enriching uranium at a high-level,” Koh said.

South Korea saw no signs of an imminent nuclear test in the North following two ballistic missile launches on March 26, Defense Ministry spokesman Kim Min Seok said that day. Six-party nuclear talks that also included China, Japan, Russia and South Korea were last held in 2008.