Responding to the North's announcement, Cho Tai-young, the spokesman for the South Korean Foreign Ministry, said in a statement that "North Korea should bear in mind that if it ignores the stern demand from the neighboring countries and the international community and carries out a nuclear test, it will have to pay a price for it."
The U.N. Security Council last week warned the North that it could face more censure because of its missile tests in the past weeks that flouted Security Council resolutions banning North Korea from testing ballistic missile technology.
The most recent test took place Wednesday, when North Korea test-fired two Rodong midrange ballistic missiles. The projectiles flew 403 miles from mobile launching pads on the west coast and landed in the sea between North Korea and Japan, South Korean officials said.
North Korea "would not rule out a new form of nuclear test for bolstering up its nuclear deterrence," the North Korean statement on Sunday said, and it accused the U.S. of "acting rashly" at the Security Council.
South Korea and international analysts have recently said that satellite imagery showed continuing activities at the North's nuclear and rocket test sites, but they reported no signs that a test was imminent. The Defense Ministry of South Korea said that a new nuclear test by North Korea was a "political" rather than technical decision, with its engineers ready to conduct one on relatively short notice from its leader, Kim Jong Un.
North Korea is also running an uranium enrichment program, first unveiled in 2010, that officials and analysts in the region fear will provide the country with a steady supply of fuel for nuclear bombs. After the North's last underground nuclear test in February last year, analysts could not determine whether the North used highly enriched uranium for fuel. The country's two previous tests in 2006 and 2009 were believed to have used some of its small stockpile of plutonium.
In a paper published on the website 38 North earlier this month, Jeffrey Lewis, a nuclear expert at the James Martin Center for Nonproliferation Studies at the Monterey Institute of International Studies, cited a "disquieting possibility": He said that recent excavation activities at the Punggye-ri nuclear test site in northeast North Korea might represent an effort to build a network of tunnels to conduct multiple tests or carry out tests on a much more regular basis using a steady supply of highly enriched uranium.
"North Korea may soon have access to regular amounts of fissile material if it doesn't already," Lewis wrote, referring to the North's uranium program. "And, although China may have pressured North Korea to refrain from nuclear tests in past years, its influence on Pyongyang seems to have waned."
North Korea said its recent missile tests were a justified reaction to the joint military exercises being carried out by South Korea and the U.S., which it said raised the danger of war.
North Korea has claimed to have developed "diversified" and "miniaturized" nuclear devices, but Western experts remain unsure about how close it has come to mastering the technology needed to mount a nuclear warhead on an intercontinental ballistic missile.
In late February, inter-Korean tensions eased enough for the two Koreas to allow hundreds of elderly Koreans separated by the Korean War six decades ago to hold brief reunions.
But since then the North has resumed its harsh language, calling President Park Geun-hye of South Korea "ignorant" and "vulgar" after she attended a global nuclear security summit meeting in the Netherlands last week and joined President Barack Obama and Prime Minister Shinzo Abe of Japan in calling on North Korea to give up its nuclear weapons.
Park repeated her appeal to North Korea in a speech in Germany on Friday, offering to funnel South Korean investments into the North's struggling agriculture and mining industries, as well as its transport and telecommunications networks, if the North ended its nuclear program.
The North has not directly responded to the overture yet, but kept up its tough talk. On Sunday, its main party-run newspaper Rodong Sinmun quoted a soldier who called Park a lunatic whose "smile hides her venom."