Seoul - 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 adopted international sanctions.

In a statement published by Pyongyang's state news agency and attributed to the National Defense Commission, the supreme military policymaking body, the North 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 stakes for the Obama administration, which has failed to curtail the North's weapons program despite a series of sanctions and short-lived attempts at dialogue.

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

But the statement Thursday was the clearest sign yet of its intentions, and it came with an unusually explicit focus on the United States, which it described as "the sworn enemy of the Korean people." The North, in raising the prospect of another nuclear test, did not say when it might be carried out.

The North added that 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, the North also pledged to launch a series of 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 North, those analysts say, is indeed becoming more dangerous.

The rocket it sent into orbit on 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 hasn't yet 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 that nuclear tests are essential for any country that wants to miniaturize its nuclear devices.

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

The North's next test could use leftovers from that stash, or it could instead use a new material: highly enriched uranium. In 2010, North Korea officials unveiled a new 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 the North 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 anywhere from zero to 11 nuclear weapons.

The North has not yet 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 young Kim appears as a smiling man of the people, but in practice, he has doubled down on the strange brand of family-run brinksmanship - all while maintaining the surveillance networks and the labor camps where some 200,000 of his people 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.

Following the North's statement Thursday, China urged for 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 U.N. Security Council deliberations, China agreed to support tougher sanctions against the North, known as Resolution 2087. With those sanctions the United Nations condemned the North's Dec. 12 rocket launch and also reasserted that the North not proceed with further launches or nuclear tests. The latest sanctions also take aim at key members 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 will likely support another round of sanctions after 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."