By proceeding with its nuclear test about 50 miles from the Chinese border, North Korea dealt a rare setback to President Xi Jinping, who had hoped a charm offensive would bring stability to the North.

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BEIJING — North Korea’s test of a nuclear bomb Wednesday seemed aimed at antagonizing a familiar adversary, the United States. The army spoke of the need to ward off “imperialist aggressors,” and a television commentator warned that foreigners were intent on destroying the country’s way of life.

But North Korea’s decision also had a more surprising target: China, its neighbor and chief ally for six decades, which had recently sought to forge closer ties with its reclusive neighbor.

By proceeding with its nuclear test about 50 miles from the Chinese border, North Korea took a gamble that China, its largest trading partner and economic lifeline, would not react with austere sanctions, as many other countries have promised.

In doing so, it dealt a rare setback to President Xi Jinping, who had hoped a charm offensive during the past six months would bring stability to the North.

“North Korea is just thumbing its nose at China,” said Douglas Paal, director of the Asia program at the Carnegie Endowment for International Peace in Washington. “That’s got to produce a reaction.”

As recently as October, China had received assurances from North Korea that it would stop its nuclear tests. It had agreed to send a senior official to a military parade in the North Korean capital, Pyongyang — the first such visit since Kim Jong Un, the current leader, took power in 2011 — partly on the condition that the North would refrain from nuclear tests, several U.S. diplomats said.

On Wednesday, officials in Beijing were furious.

“China strongly opposes this act,” Hua Chunying, a spokeswoman for the Chinese Ministry of Foreign Affairs, said in unusually harsh remarks at a news conference Wednesday. “China will firmly push for denuclearization of the Korean Peninsula.”

Xi is in a difficult position. He is under intense pressure to inflict harsh economic punishment on North Korea, but many Chinese worry that any resulting instability could seep back into their territory. He also faces new questions about China’s efforts to curry favor with Kim, whom many Chinese regard as a bizarre, bumbling figure.

Xi must also reckon with the prospect that actions by the North could galvanize countries such as the United States, Japan and South Korea into strengthening military forces in the Pacific, just as China is seeking to assert its dominance in the region.

“This is precisely what China does not need,” said Rory Medcalf, head of the National Security College at Australian National University. “If there is one issue guaranteed to refocus U.S. strategic attention on Asia, it is a North Korean nuclear provocation, especially one purported to bring Pyongyang’s destructive capabilities to a whole new level.”

Chinese officials sidestepped questions Wednesday about whether they would level economic sanctions against North Korea, by paring shipments of oil, for instance, or food. China is North Korea’s largest economic ally, with trade between the two countries having totaled $6.4 billion in 2014.

Cheng Xiaohe, a professor of international relations at Renmin University in Beijing, said China had several ways of increasing economic pressure, including by limiting the number of tourists allowed to visit North Korea or by reducing Chinese imports of commodities such as coal.

But he said the leadership in Beijing should be cautious before resorting to more drastic measures.

“If China unilaterally imposes sanctions, it will have no more tricks,” Cheng said. “Putting more economic pressure on North Korea might also lead to the fall of Kim, the collapse of the regime and all kinds of unpredictable situations China does not wish to see.”

Other nations’ response to North Korea’s claim that it had tested a hydrogen bomb for the first time was not as muted as China’s, with other world powers vowing to punish the country with new international sanctions.

The isolated country’s fourth nuclear test since 2006 was a “reckless challenge to international norms of behavior and the authority of the U.N. Security Council,” said British Ambassador Matthew Rycroft.

The council met in an emergency session and called the test “a clear violation” of its resolutions. It agreed to start work immediately on a resolution for new sanctions.

The claim that a hydrogen bomb had been successfully tested also was met with skepticism.

An early analysis by the U.S. government was “not consistent with the claims that the regime has made of a successful hydrogen-bomb test,” White House spokesman Josh Earnest said.

He added that nothing had happened in the last 24 hours to change the U.S. assessment of North Korea’s technical or military capabilities. The U.S. is still doing the work needed to learn more about the North’s test, he added.

Hours earlier, South Korea’s spy agency said it thought the estimated explosive yield from the blast was much smaller than what even a failed hydrogen-bomb detonation would produce.

South Korean lawmaker Lee Cheol Woo said he was told in a briefing by the National Intelligence Service that the North may not have conducted a hydrogen-bomb test given the relatively small size of the seismic wave reported.

An estimated explosive yield of 6.0 kilotons and a quake with a magnitude of 4.8 (the U.S. reported 5.1) were detected, Lee said he was told. That’s only a fraction of the hundreds of kilotons that a successful H-bomb test would usually yield.

“I’m pretty skeptical,” said Melissa Hanham, senior researcher at the James Martin Center for Nonproliferation Studies at the Middlebury Institute for International Studies in Monterey, Calif. “The seismic data indicates it would be very small for a hydrogen test. “It seems just too soon to have this big technical achievement,” she said. “But North Korea has always defied expectations.”

While also noting the quake was likely too small for an H-bomb test, Jaiki Lee, a professor of nuclear engineering at Seoul’s Hanyang University, said the North could have experimented with a “boosted” hybrid bomb that uses some nuclear-fusion fuel along with more conventional uranium or plutonium fuel.

Joel Wit, founder of the North Korea-focused 38 North website, said a boosted bomb “is the most likely option,” while adding that he isn’t surprised that North Korea has shifted focus to hydrogen weaponry.

Fusion is the main principle behind the hydrogen bomb, which can be hundreds of times more powerful than atomic bombs that use fission. In a hydrogen bomb, a nuclear-fission explosion sets off a fusion reaction responsible for a powerful blast and radioactivity.