After North Korea's claim that it tested a hydrogen bomb, The Post's Karen DeYoung explains why the U.S. response to the rogue nation is so different from how it treats Iran. (Jason Aldag,Gillian Brockell/The Washington Post)

After North Korea's claim that it tested a hydrogen bomb, The Post's Karen DeYoung explains why the U.S. response to the rogue nation is so different from how it treats Iran. (Jason Aldag,Gillian Brockell/The Washington Post)

The United States and other global powers sharply condemned North Korea on Wednesday, vowing to punish it for conducting a fourth nuclear weapons test. But it was not clear what more the world could do, short of war, to a country that for years has been impervious to international isolation and sanctions.

The Obama administration and nuclear experts moved quickly to question North Korea’s claims that it had tested a hydrogen bomb, a massively powerful thermonuclear device that would mark a major advance in Pyongyang’s weapons ability.

Although there is more investigation to be done, administration spokesmen said, initial seismic and other data indicated that the explosion was more likely to have been a far smaller atomic weapon along the lines of North Korea’s previous tests.

[A look behind the North’s claim]

The United States and Japan called for an emergency meeting of the United Nations Security Council. Following the closed-door session, diplomats said they would move toward a new council resolution to increase economic and diplomatic pressure on North Korea.

1 of 9 Full Screen Autoplay Close Skip Ad × These countries have tested nuclear weapons View Photos Since 1945, eight nations have detonated such weapons. Caption Since 1945, eight nations have detonated such weapons. North Korea North Koreans watch a news broadcast on a video screen outside Pyongyang Railway Station in Pyongyang, North Korea, on Jan. 6, 2016. The nation claims that it has successfully tested a hydrogen bomb, but the claim drew skepticism. Kim Kwang Hyon/AP Buy Photo Wait 1 second to continue.

“The international community must impose real consequences for [North Korea’s] destabilizing actions and respond with steadily increasing pressure,” Samantha Power, the U.S. ambassador to the United Nations, said after the meeting. “The Security Council’s commitment today to impose ‘further significant measures’ . . . marks an important step in that process.”

North Korea’s three previous nuclear tests since 2006 have been met with international condemnation, including resolutions and sanctions from the Security Council. But the response has done nothing to deter Pyongyang. In a statement announcing the test, the official Korean Central News Agency said the weapon was needed for defense against the United States, which it described as “the chieftain of aggression” and a “gang of cruel robbers.”

Any additional pressure on North Korea — already the most severely sanctioned country in the world — would have to involve major action by China, its largest trading partner and supplier of much of its foreign aid. The Chinese government expressed strong opposition to the North Korean test and called on its government to cede to international demands that it eliminate its nuclear program.

[North Korea also tests limits of Chinese ties]

But Beijing has been reluctant to risk action that could seriously destabilize North Korea, sending hordes across its border, perhaps leaving nuclear weapons unsecured, and ceding a strategic bulwark against the powerful U.S., Japanese and South Korean military alliance in the northern Pacific.

To some degree, China has opted to stick with “the devil you know,” said Jeffrey A. Bader, a former senior Obama adviser on Asia on the National Security Council and current senior fellow at the Brookings Institution.

Attempts to impose further restraints on North Korea’s nuclear supply line are viewed as likely to have only modest, if any, effect. The country received most of what it needed from abroad to develop nuclear weapons years ago, and its program is now mostly indigenous.

One avenue for further economic pressure would be to sanction Chinese banks that do business with North Korea, although there is widespread reluctance to take measures against major Chinese institutions that are pillars of the international financial system. Smaller banks, however, could be sanctioned, experts said.

The administration acknowledged that China would have to play a significant role in any additional action against North Korea, and it cited what White House press secretary Josh Earnest said was Chinese President Xi Jinping’s agreement, during his visit with Obama last September, that neither of them would accept North Korea as a nuclear power.

“There are some disagreements” between the United States and China, Earnest said, “but this is an issue on which we do agree.” The international community is united, he said, on the need for North Korea to cease its nuclear provocations and “commit to the denuclearization of the Korean peninsula.”

[Watch: North Korea’s broadcast]

Insistence that there was nothing to talk to Pyongyang about until it gave up its nuclear weapons and submitted to international verification has been the foundation of Obama’s North Korea strategy, and the reason there have been no talks with its government for nearly a decade. North Korea has said it will only discuss its weapons program as a full partner in the nuclear club.

That, U.S. officials repeated Wednesday, is unacceptable. “We do not and will not accept North Korea as a nuclear armed state, and actions such as this latest test only strengthen our resolve,” Secretary of State John F. Kerry said in a statement.

Earnest said Obama and his national security team would be exploring “the appropriate way to respond” to the new test but would not discuss its parameters in advance.

He was dismissive of Republican presidential candidates and other political opponents who were harshly critical of Obama’s North Korea policy and called for more pressure. “They’re trying to win votes from Republicans in a presidential primary,” Earnest said. “I’ve heard a lot of campaign rhetoric but not a lot of specific” proposals about what to do.

Bader, who advised Obama on both North Korea and China policy, said: “I don’t see how [North Korean aggression] can be attributed to Obama weakness. Obama’s been harder” on Pyongyang than the George W. Bush administration, he said, “by any measure.”

Bush’s efforts to negotiate with North Korea, Bader said, “led to [North Korea’s] first nuclear test in 2006.

“Some things are not because of what we do,” he said. North Korea has had “a strategic plan to develop nuclear weapons for more than 20 years, and there are only two ways to change that. One is regime change, something that we all hope is going to happen but we can’t engineer. Number two is a negotiated agreement. Bush tried that, but it collapsed.”

North Korea’s test appeared to be part of efforts by its leader, Kim Jong Un, to project strength as the country faces increasing isolation. Its renegade status has been further underscored by the nuclear deal reached last year between Iran and world powers, including the United States and China. Under the pact, Tehran agreed to limits on its nuclear program in exchange for the easing of international sanctions.

Over the past decade, Iran has developed capacities to make nuclear material, but its leaders insist they do not seek an atomic weapon.

In contrast, North Korea “is apparently willing to accept international isolation,” said Earnest.

As international experts continued to assess the explosion, Lassina Zerbo, executive secretary of the Comprehensive Nuclear-Test-Ban Treaty Organization in Vienna, said a seismic reading on Wednesday was “slightly down” compared with the level from the North’s last nuclear test in 2013.

A full analysis by the watchdog group could take days as experts look at other data including airborne radioactive isotopes, Zerbo said.

In Seoul, South Korean President Park Geun-hye put her military on alert and said North Korea would pay a price for the test, which she called a “grave provocation.” Japanese Prime Minister Shinzo Abe had a similar message, describing it as “a major threat” that Japan “absolutely cannot accept.”

Russia, which declared 2015 a “year of friendship” with North Korea, also condemned the detonation and called for international nuclear talks with North Korea to resume.

Russian President Vladimir Putin, who gave Kim a rare foreign invitation for a visit last year, ordered a full study of data from Russia’s monitoring stations, said the spokesman, Dmitry Peskov.

Fifield reported from Tokyo. Simon Denyer in Beijing, Michael Birnbaum in Moscow, Yoonjung Seo in Seoul, Yuki Oda in Tokyo and Carol Morello in Washington contributed to this report.

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