This picture taken by North Korea's official Korean Central News Agency on Dec. 12, 2012, shows North Korean leader Kim Jong-Un celebrating with staffs from the satellite control center during the launch of the Unha-3 rocket, carrying the satellite Kwangmyongsong-3, at the general satellite control and command center in Pyongyang. (AFP/AFP/Getty Images)

North Korea on Thursday threatened to carry out a nuclear test as part of an “all-out action” against the United States, which it called the “main player” behind recently tightened international sanctions.

In a statement published by Pyongyang’s state news agency and attributed to the National Defense Commission, the supreme military policymaking body, North Korea said Washington’s policy toward it had entered “a new dangerous phase.”

Although an underground nuclear test would not directly threaten the United States, it would raise the stakes for the Obama administration, which has been unable to curtail the North’s weapons program despite sanctions and short-lived attempts at dialogue.

White House press secretary Jay Carney said the North’s threats are “needlessly provocative.”

“We judge North Korea by its actions,” Carney told reporters at his daily briefing. “Provocative acts like this are significant violations.”

Intelligence experts in Seoul and Washington have speculated for months that the secretive police state is preparing to conduct its third nuclear test, based on satellite photos showing activity at the North’s test site. Pyongyang’s state news agency also has made several opaque references about bolstering the nation’s “nuclear deterrent.”

The statement Thursday was the clearest sign yet of the North’s intentions, though it did not say when the threatened nuclear test might be carried out. The statement also had an unusually explicit focus on the United States, which Pyongyang described as “the sworn enemy of the Korean people.”

North Korea said it would retaliate against the United States with “force, not with words, as it regards jungle law as the rule of its survival.” As part of this show of force, it also pledged to launch long-range rockets, similar to the one it sent into orbit last month, which prompted the toughened U.N. Security Council sanctions.

“They have been hinting at [a nuclear test], I suppose, for some time,” said Glyn Davies, the Obama administration’s envoy for North Korea policy, who was in Seoul on Thursday. “We think that that would be a mistake, obviously. We call on North Korea, as does the entire international community, not to engage in any further provocations.”

North Korea has spent decades as East Asia’s chief provocateur — developing weapons, launching rockets, making and breaking denuclearization deals, threatening all-out war — and analysts admit that its rhetoric can often feel repetitive. But the country, the analysts say, is indeed becoming more dangerous.

The rocket that Pyongyang sent into orbit Dec. 12, according to South Korean analysis, was made largely with indigenous components and could be capable of reaching the United States. Although North Korea has not shown the ability to miniaturize a nuclear weapon enough to mount on a rocket, some security analysts say the country could hone such technology within several years. Scientists say nuclear tests are essential for any country that wants to miniaturize its nuclear devices.

If the country conducts another nuclear test, it would lend new clues about its weapons material. North Korea’s first two nuclear tests, in 2006 and 2009, used plutonium, and the country — though it has idled its plutonium program — still has about 24 to 48 kilograms on hand, enough for four to eight bombs, according to Siegfried Hecker, a former director of the Los Alamos National Laboratory.

The next test could use leftovers from that stash, or it could use a new material: highly enriched uranium. In 2010, North Korean officials unveiled a 2,000-centrifuge uranium enrichment plant to a small team of foreign visitors that included Hecker. The North Koreans said their new program was for peaceful purposes only; that is, they planned to produce low-enriched, not high-enriched (or weapons-grade), uranium.

Many intelligence experts, though, speculate that North Korea is doing otherwise and has additional, clandestine uranium facilities throughout the country. A report last year from the Washington-based Institute for Science and International Security, acknowledging the many unknowns, said the North could have enough weapons-grade uranium for up to 11 nuclear weapons.

North Korea has not conducted a nuclear test under supreme leader Kim Jong Eun, who inherited power when his father, Kim Jong Il, died in December 2011. In state propaganda, the younger Kim is depicted as a smiling man of the people, but in practice, he has doubled down on the strange brand of family-run brinkmanship — all while maintaining the surveillance networks and the labor camps in which about 200,000 North Koreans are imprisoned.

“Kim Jong Eun seems to have concluded it is advantageous to be armed with nuclear weapons” to show off national strength, said Kim Heung-kyu, a professor of politics and diplomacy at Sungshin Women’s University in Seoul.

After North Korea’s statement Thursday, China urged calm from all involved parties, a familiar talking point from Pyongyang’s chief economic partner. Another nuclear test would present a particular challenge for new Chinese leader Xi Jinping and his lieutenants, who are torn between supporting a key ally and maintaining international credibility.

In the latest Security Council deliberations, China agreed to support a resolution tightening sanctions against the North. Resolution 2087 also condemned the North’s Dec. 12 rocket launch and reasserted that the North not proceed with further launches or nuclear tests. The latest sanctions also take aim at key figures and trading corporations involved in North Korea’s space program, freezing assets and attempting to stop the trade of weapons technology.

Resolution 2087 also promises “significant action in the event of a further [North Korean] launch or nuclear test.”

Zhu Feng, a professor of international relations at Peking University in Beijing, said China is likely to support another round of sanctions if the North conducts a third nuclear test.

“China has no option but to support the U.N.,” Zhu said. “Under such a situation, Kim Jong Eun should make serious considerations about the consequences. Anther nuclear test by North Korea could heighten the unrest in Northeast Asia.”

Scott Wilson and Karen DeYoung in Washington, Yoonjung Seo in Seoul and Liu Liu in Beijing contributed to this report.