BEIJING >> North Korea had detonated a nuclear bomb, and the president of China was urging caution. It was the fall of 2013, and the North’s third nuclear test in seven years, carried out several months earlier, had rattled much of the world.

But President Xi Jinping, in a private meeting with President Barack Obama at Constantine Palace in St. Petersburg, Russia, warned against putting too much pressure on Kim Jong Un, the North’s young, volcanic leader.

“A barefoot person does not fear those who wear shoes,” Xi told Obama, invoking a Chinese proverb to convey that an impoverished nation like North Korea had nothing to lose by standing up to China and the United States. The conversation was recounted by an American diplomat familiar with the talks, who spoke on the condition of anonymity for fear of angering the Chinese.

Since coming to power in 2012, Xi has pushed the limits of Chinese foreign policy, challenging U.S. influence in the Pacific and using China’s financial heft to win allies across the globe. But while Xi has taken a tougher approach than his predecessors on North Korea, he has resisted inflicting crippling punishments on the North, an ally for six decades and a valuable counterweight for Beijing to U.S. military might in Asia.

After North Korea conducted its fourth nuclear test last week, world leaders escalated pressure on Xi, who many see as the best hope of reining in Kim. South Korea’s president, Park Geun-hye, who has cultivated closer ties to Xi, called on China this week to match its disapproving words about the North’s nuclear ambitions with “necessary measures.”

But Chinese scholars and officials involved in North Korean policy said that Xi was reluctant to take sweeping action, by cutting shipments of oil and food, for instance, or blocking access to banks. He has not wavered from his view, expressed to Obama in 2013, that destabilizing the North would create chaos in the region, these people said. And he is especially sensitive to the prospect of a reunified Korea backed by the United States, at a time when China is trying to assert its dominance in Asia.

“If North Korea becomes an enemy state, it would have plenty of ways to harm China,” said Shi Yinhong, a professor of international relations at Renmin University in Beijing. “Beijing cannot afford to have North Korea become permanently hostile.”

Adding to the complications, Xi, 62, and Kim, who is believed to be 33, have a fraught relationship, according to American, Chinese, and South Korean officials.

Xi has traveled to dozens of countries as president, but he has yet to make the two-hour flight to Pyongyang, the North Korean capital, once a rite of passage for Chinese leaders. Kim declined an invitation to attend a military parade in Beijing in September, and he hastily called off an appearance in Beijing last month by a North Korean pop band.

While the two leaders hail from revolutionary families, they have little else in common. Xi’s formative years were dominated by the chaos of the Cultural Revolution. Kim attended a Swiss boarding school and inherited the title of supreme leader before turning 30.

“Xi has been through a very exacting training program where status comes from age,” said Kerry Brown, a former British diplomat who is now director of the Lau China Institute at King’s College London. “Here we have this guy who is 33, who has been to Geneva for a couple years, who has got no executive experience whatsoever, basically running this parasitical economy, which isn’t functioning very well.”

Xi has long recognized China’s sense of camaraderie with the North, which dates to their Korean War alliance in the 1950s. (Mao called the relationship “as close as lips and teeth.”) In a speech in 2010, when Xi was vice president, he called that conflict “a just war” and spoke of a “friendship forged by blood.” His father, Xi Zhongxun, served as an envoy to North Korea for the Chinese leader Deng Xiaoping in the 1980s and had a close relationship with Kim Il Sung, the North’s founding leader and Kim Jong Un’s grandfather.

But the two countries have followed starkly divergent paths. While China is now a sophisticated economy and a rising power, North Korea has become increasingly isolated, enfeebled and erratic, depending on China for most of its food and energy.

Yang Xiyu, a former senior Chinese official who oversaw negotiations with North Korea, said the chance of a meeting between the two leaders, which was discussed privately by Chinese officials last year, was now “sharply reduced.”

“The nuclear test will seriously damage the bilateral relationship,” Yang said. “Xi Jinping has been forced to be more assertive.”

While his predecessors welcomed North Korean leaders with the fanfare of Politburo meetings, Xi has kept a distance. Chinese and American officials trace that to his distrust of Kim, whose first nuclear test as supreme leader came in February 2013, just a few months after Xi came to power.

In an unusually public rebuke, Xi warned that no country should be able to throw the world into chaos for “selfish gain.” Later that year, he imposed sanctions, limiting shipments of materials used in weapons and cutting ties to some North Korean banks, though enforcement was lax.

Xi has made clear to the North that its future lies in economic reform, not military development, and that China will not accept a nuclear state, current and former Chinese and U.S. officials said. Increasingly, he has come to see the North as a liability, at odds with his vision of making China a pre-eminent superpower.

“Kim has kept challenging China’s fundamental interests, policies and the security of the whole northeast of Asia,” Shi said.

In a sign of his displeasure, Xi has cultivated better relations with Park, the South Korean president, traveling to Seoul for a state visit in 2014. Those efforts seem in part aimed at undermining U.S. power in Asia. Beijing considers Seoul the “weakest link” among U.S. allies in the region, said Seong-Hyon Lee, an assistant professor at Kyushu University in Japan.

Privately, officials at the Chinese Foreign Ministry have become more vocal about their distrust of the North and Kim, as unease among the Chinese public about the country’s erratic ally has grown, American diplomats said.

Jon M. Huntsman Jr., who served as the U.S. ambassador to China from 2009 to 2011, said there was a generational divide among Chinese officials about how to deal with North Korea.

“The older apparatchiks would defend the North Korean line,” he said. “The younger ones wanted this issue to go away. There’s no emotional connection, there’s no war being waged.”

In recent months, Xi extended several olive branches to Kim, concerned that the relationship had deteriorated to the point that Kim might lash out again, American and Chinese diplomats said. In October, Xi dispatched a top official to a military parade in Pyongyang, carrying a letter from Xi extolling Kim’s achievements, which some officials viewed as a precursor to a meeting between the two leaders.

John Delury, an associate professor at Yonsei University in Seoul, said those efforts showed that Xi could accept Kim’s leadership of North Korea.

“But what he needs and expects from Kim is to show the kind of deference that Korea, a ‘small country,’ owes China, a ‘great power,’” Delury wrote in an email. “Kim, for his own reasons, refuses to give it.”

At the parade in October, Kim stood next to Xi’s envoy, smiling and waving. He spoke of a “blood-tied friendship” and said that “bilateral ties are more than neighborliness,” according to coverage in the North Korean news media.

“Traditions should not be confined to history books,” Kim said. “They should be carried out in practice.”

In its report on the visit, Xinhua, the official news agency of the Chinese Communist Party, omitted Kim’s remarks.