North Korea has threatened to carry out a “new form” of nuclear test, further escalating tensions with South Korea, which warned that Pyongyang will “pay the price” for such a move.

North Korea has not explained what form such a test would take, but U.S. officials have long suspected that the communist regime has sought to build “nuclear devices small and sophisticated enough to be delivered by intercontinental ballistic missiles it was also developing,” the New York Times reported.

The North Korean Foreign Ministry warned that the nation’s forces would hold exercises aimed at bolstering its ability to carry out military strikes and said its military sought the capability to  attack mid- and long-range enemy targets with “more diversified nuclear deterrence.”

The latest threats follow joint military exercises carried out by U.S. and South Korean forces – actions Pyongyang claims raise the danger of war. South Korean officials warned Monday that Seoul would retaliate if North Korean forces fired south of the maritime border between the two nations, which is in dispute.

Last week the U.N. Security Council warned North Korea it could face condemnation for  flouting existing council resolutions barring it from testing ballistic missile technology.
The most recent test took place on Wednesday, when North Korea fired two  medium-range Rodong ballistic missiles which flew more than 400 miles from its western coast and landed in the sea between Japan and North Korea.

Pyongyang denounced Washington for behaving “rashly” at the Security Council and said Sunday that it “would not rule out a new form of nuclear test for bolstering up [sic] its nuclear deterrence.”

South Korean Defense Ministry officials say North Korean engineers stand prepared to conduct a nuclear weapons test on short notice when ordered to by the country’s strongman, Kim Jong Un.

North Korea carried out nuclear tests in 2006, 2009 and 2013. The first two were believed to have used plutonium.

Four years ago, North Korea unveiled a uranium enrichment program, but it is unclear whether its most recent test in February 2013 utilized uranium.

South Korean officials worry that the uranium enrichment program will give Pyongyang a consistent fuel supply for atomic bombs.

In a paper published last month, Jeffrey Lewis, a nuclear expert at the James Martin Center for Nonproliferation Studies, expressed concern that recent excavation at a site in northeastern North Korea might be part of an effort to build a tunnel network. It could be used  to conduct tests on a more regular basis using highly enriched uranium.

Referring to North Korea’s uranium program, Lewis wrote that the nation “may soon have access to regular amounts of fissile material if it doesn’t already,” adding that it is unclear whether Beijing will use its influence to dissuade Pyongyang from additional provocations.

Even though “China may have pressured North Korea to refrain from nuclear tests in past years, its influence on Pyongyang seems to have waned,” Lewis said.

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