BEIJING – Hours after carrying out a nuclear test in defiance of international warnings not to, North Korea warned Tuesday that it will take new unspecified actions if the United States doesn’t curb its hostility toward the rogue nation.

In a statement it attributed to the country’s Foreign Ministry, the North Korean state news agency blamed the United States for U.N.-imposed sanctions intended to discourage North Korea’s missile and nuclear development programs. A continuation of that approach, the statement said, would leave North Korea “with no option but to take the second and third stronger steps.”

South Korea’s intelligence service warned that its northern neighbor might conduct another nuclear test and might repeat its December launch of another long-range missile.

The threats came as scientists began analyzing data from the test to determine the strength of the explosion and what it might portend about the country’s ability to accumulate a nuclear arsenal. The U.N. agency that’s charged with monitoring nuclear tests around the world said Tuesday’s blast was at least twice as powerful as one the North Koreans set off in 2009 and “much larger” than the country’s first test, in 2006.

North Korea official state news media described the device as “small and light,” characteristics that seemed to indicate it could be used atop a rocket or missile, a step that would put countries such as Japan and even parts of the United States within range.

Meeting in New York, the U.N. Security Council condemned the test, calling it “a clear threat to international peace and security.” President Barack Obama also denounced the test, as did a wide range of foreign governments.

U.S. Rep. Ed Royce, R-Calif., the chairman of the House Foreign Affairs Committee, called on the Obama administration to take tougher steps against North Korea, noting that the country also had launched a multi-stage rocket in December and had posted a bizarre video on the Internet that shows a sleeping North Korean dreaming of a nuclear attack on New York.

“The administration must replace its failed North Korea policy with one that is energetic, creative and focused on crippling the Kim regime’s military capabilities,” Royce said in a statement, referring to North Korean leader Kim Jong Un. “Otherwise, the grave North Korean threat to the region and the United States will only grow.”

It wasn’t at all clear what those tougher steps might be. The United States and the United Nations already have placed North Korea under a wide range of economic sanctions, and stronger steps, such as a naval blockade, would need the agreement of North Korea’s principal ally, China.

How willing _ and able _ China would be to impose tougher measures remains to be seen. China repeatedly has admonished its neighbor from conducting such a test, apparently to no avail.

Chinese Foreign Minister Yang Jiechi summoned the North Korean ambassador to Beijing, warning him that China was “strongly dissatisfied” with the nuclear test and calling on Pyongyang to refrain from actions that might worsen the situation, according to a ministry news release.

But the wording of a statement the Foreign Ministry issued in Beijing condemning North Korea’s actions mirrored that of 2009, when North Korea staged a previous test and wasn’t cast away by China. It made no threat of sanctions or other consequences and urged “all parties to respond in a cool-headed manner” through dialogue.

State Department spokeswoman Victoria Nuland said in Washington that China would be crucial in sending a message to North Korea.

“The Chinese have the most influence. … That’s obvious, given their well-intermeshed economic relationship with the DPRK,” Nuland said, using the initials for North Korea’s official name, the Democratic People’s Republic of Korea. “That’s why, among other reasons, it’s so important for us to stay closely linked up with China.”

But whether China would take a tougher stance _ moving to cut off oil supplies, for example _ was uncertain. Maintaining the status quo in North Korea is important to China, not only because it provides a buffer between China and U.S. soldiers in South Korea, but also because it prevents the chaos of a destabilized nation spilling over its borders.

“It’s unrealistic for us to expect a change overnight,” Zhu Feng, a noted expert on international relations at Peking University, said in a telephone interview Tuesday night. But this most recent round of trouble, Zhu said, could well prove an impetus for Beijing to reconsider its policy.

It also remained far from clear what Pyongyang ultimately hoped to accomplish. North Korea has long been characterized by erratic and secretive actions, and Kim, who’s thought to be about 30 years old, might be fueling the standoff to solidify his domestic standing. He was named the nation’s ruler in December 2011 after the death of Kim Jong Il, and some observers wondered whether the son was up to the task of succeeding his father.

In the past, North Korea also has used intensely provocative behavior to gain attention on the global stage before making the case for more aid to the impoverished nation.

There were hints of that history in Tuesday’s statement from North Korea. Mixed into the bombastic language about new North Korean actions was another option: The United States could “open a phase of detente and stability.” North Korea tied that scenario to American acceptance of Pyongyang’s “right to satellite launch,” a reference to a launch in December that was widely considered a test of its missile program.

Adding to the mystery was that North Korea had notified the United States that the test was coming. The United States attempted to persuade the North Koreans not to proceed, spokeswoman Nuland said, without success.

Nuland declined to specify how much advance notification the North Koreans had provided, saying only that the contact was made recently.

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International nuclear experts were trying Tuesday to determine just what North Korea had exploded and what it would say about the country’s nuclear capabilities.

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South Korea’s National Intelligence Service told members of the nation’s intelligence committee that “it is too early to say the North has succeeded in weaponizing its nuclear technology,” according to that country’s Yonhap news agency.

The Office of the Director of National Intelligence in Washington was circumspect in its comment on the test. “North Korea probably conducted an underground nuclear explosion” with a yield of “approximately several kilotons,” it said in a statement. “Analysis of the event continues.”

James Acton, a physicist and nuclear weapons expert with the Carnegie Endowment for International Peace, a policy institute in Washington, said there was “very little doubt in my mind that this was a nuclear explosion,” and the office of U.N. Secretary-General Ban Ki-moon issued a statement referring to the event as an “underground nuclear weapon test.”

“The secretary-general is gravely concerned about the negative impact of this deeply destabilizing act on regional stability as well as the global efforts for nuclear nonproliferation,” the statement said. It said that Ban once again urged North Korea “to reverse course and work towards denuclearization of the Korean peninsula.”

Two key questions about the blast will be what kind of device was used and whether North Korean scientists have been able to produce sufficient amounts of highly enriched uranium from a facility that the secretive Stalinist regime revealed only in 2010.

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North Korea’s two previous nuclear tests involved plutonium recovered from spent nuclear-reactor fuel rods, a source of explosive material that would limit the country’s likely arsenal to perhaps a dozen weapons.

But if North Korea has refined enough uranium to produce a weapon, the supply of fuel would be unlimited, as long as the country could obtain supplies of unprocessed uranium.

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The Comprehensive Nuclear-Test-Ban Treaty Organization, the Vienna-based U.N. agency that’s charged with monitoring nuclear tests, said in a statement that its monitoring stations around the world had detected an explosive shock wave emanating from North Korea at about 9:57 p.m. EST Monday _ 11:57 a.m. Tuesday in Korea.

It pinpointed the explosion’s location at about the same place in North Korea where the country had conducted its first two nuclear tests, and it reported that Tuesday’s blast appeared to be far more powerful than the first two.

The U.S. Geological Survey estimated the strength of the explosion at 5.1 on the Richter scale, the measure used for earthquakes, a figure that suggested the explosion had a yield of 3 to 10 kilotons of TNT. The U.S. atomic bomb that was dropped on Hiroshima, Japan, near the end of World War II had a yield of about 16 kilotons.

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(Allam reported from Washington. Jonathan S. Landay and Mark Seibel contributed to this report from Washington.)

• Tom Lasseter and Hannah Allam, MCT Campus