WASHINGTON — When Wang Yi, China’s foreign minister, called on Saturday for talks to resolve the crisis in Europe, he said Ukraine’s sovereignty should be “respected and safeguarded” — but also sided with Russia in saying that NATO enlargement was destabilizing the continent.

“If NATO keeps expanding eastward, is it conducive to maintaining peace and stability in Europe?” he said by video at the Munich Security Conference in Germany, which Vice President Kamala Harris was attending in person to rally countries against Russia.

It was the latest instance of China taking a bold new swing at the United States and its allies by wading into European security issues to explicitly back Russia, which has amassed more than 150,000 troops around Ukraine for a possible invasion — despite the fact Ukraine is not joining NATO anytime soon.

Current and former U.S. and European officials say they are alarmed over what is effectively a nonaggression pact between China and Russia that could amount to a realignment of the world order. Portending a new type of Cold War, Biden administration officials say the United States will work to create and bolster its own coalitions of democratic nations — including new Europe and Asia-Pacific strategic groups — and help countries develop advanced military capabilities.

John F. Kirby, the chief Pentagon spokesman, said last week that the U.S. government was watching the “burgeoning relationship” between China and Russia. He said that a joint statement issued by the two countries in early February when Presidents Vladimir V. Putin of Russia and Xi Jinping of China met in Beijing showed that China was standing behind Mr. Putin’s military buildup around Ukraine.

“Their tacit support, if you will, for Russia is deeply alarming, and, frankly, even more destabilizing to the security situation in Europe,” Mr. Kirby said.

In recent weeks, the two nations negotiated a 30-year contract for Russia to supply gas to China through a new pipeline. They blocked a demand from Washington that the United Nations impose additional sanctions on North Korea for new missile tests, even though the two nations had agreed to similar sanctions before. And Russia moved large numbers of troops from Siberia to its west, a sign that Moscow, in preparing for a potential invasion of Ukraine, trusts China along their shared border in the east.

Their long courtship reached a peak with the 5,000-word joint statement that said their partnership had “no limits,” which some Biden administration officials see as a turning point in China-Russia relations and a brazen challenge to American and European power. The statement was the first in which China explicitly joined Russia in opposing any further expansion of NATO, and the two countries denounced Washington’s Indo-Pacific strategy and its new security partnership, AUKUS, which includes Britain and Australia. The nations also described Taiwan as “an inalienable part of China.”

China and Russia declared that they would work with other countries to “promote genuine democracy” and counter American-led ideology and institutions — building a new world order in which autocracies are unchallenged, U.S. and European officials say.

“They seek a new era, as they say, to replace the existing international order,” Ursula von der Leyen, the president of the European Commission, said in Munich on Saturday. “They prefer the rule of the strongest to the rule of law, intimidation instead of self-determination, coercion instead of cooperation.”

The strengthening China-Russia ties could herald a reconfiguring of the triangle of power that defined the Cold War and that President Richard M. Nixon exploited 50 years ago on Monday when he made a historic visit to Beijing to normalize diplomatic relations. That helped the United States and China counterbalance the Soviet Union. Ties between Beijing and Moscow had been unraveling for years over issues of ideology and foreign policy.

The opposite is happening now.

“It’s certainly concerning, and it is not a positive development from the standpoint of U.S. national security or U.S. national interests,” said Susan Shirk, the chair of the 21st Century China Center at the University of California, San Diego, and a former State Department official. “They have a kind of common perspective on the U.S. right now, and there is this affinity between the leaders.”

Ms. Shirk said President Biden nonetheless should try engaging in diplomacy with Mr. Xi to coax him to act with the United States on the Russia-created Ukraine crisis. “This seems like Diplomacy 101 given at least the history of this triangular relationship,” she added.

China and Russia are not united by ideology, and they are in a marriage of convenience that Russia needs more. While Mr. Xi appreciates Mr. Putin’s defiance of the United States, he does not want the economic uncertainty that a European war would bring. China also traditionally insists on respecting every nation’s sovereignty, as Mr. Wang made clear on Saturday.

There are limits to what China would do to help Mr. Putin if he invades Ukraine. After Washington imposes sanctions on Russia, Chinese companies could buy more oil and gas from Russia and help fill some technology gaps, but the major Chinese state-owned banks would probably refrain from overt violations of the sanctions for fear of being shut out of the global financial system.

Mr. Xi and Mr. Putin have met 38 times as national leaders. They share a drive to restore their nations to a former glory that they see as having been stripped from their homelands by Western European powers, the United States and, in China’s case, Japan. Both are obsessed with the breakup of the Soviet Union in 1991: Mr. Putin seeks to forcefully wind back the clock to a pre-collapse era, while Mr. Xi aims to prevent China from meeting the same fate as the Soviet empire. They accuse Washington of fomenting mass protests and democracy movements around the world to overthrow other governments.

An intensifying conflict with China and Russia would have a different shape than the Cold War. China’s trade economy is deeply integrated with those of other nations, including the United States, and Russia is an important energy exporter to Europe. For practical reasons, the three governments would be unable to completely block commercial exchanges with each other or form distinct economic blocs with partner countries, like in the days of the Iron Curtain.

Nevertheless, foreign leaders and Democratic and Republican foreign policy practitioners have expressed concern in recent days.

In a Wall Street Journal opinion column titled “Entente Multiplies the Threat From Russia and China,” John R. Bolton, the hawkish national security adviser under President Donald J. Trump, argued that the partnership “will last” because the two countries’ interests “are mutually complementary for the foreseeable future.” Stephen J. Hadley, the national security adviser under President George W. Bush, called the joint statement “a manifesto for their global leadership,” while Kevin Rudd, the former prime minister of Australia, said China’s explicitly pro-Russia position on European security was “new and significant and quite a radical departure from the past.”

Scott Morrison, the current prime minister of Australia, denounced China last week for remaining “chillingly silent” on Russia’s military buildup around Ukraine and noted that the two countries were “banding together.”

A senior U.S. official said the Biden administration would counter the two powers in part by trying to create “greater connectivity” among democratic partners and allies of the United States, one that goes beyond regional coalitions. Such an approach has been a central thrust for Mr. Biden, who during the 2020 campaign said Russia was the greatest foreign policy challenge for the United States in the medium term, and China the biggest in the long term.

Mr. Biden pushed NATO to issue a summit communiqué last June that laid out the challenges that China poses to the alliance, a position that Jens Stoltenberg, the secretary general of NATO, has reiterated. The president held a virtual “Summit for Democracy” in December in which he spoke by video with officials from more than 100 countries. And this month, the White House released an Indo-Pacific strategy paper that said the United States would promote democratic institutions among partner nations and help them “deploy advanced war-fighting capabilities,” such as helping Australia build nuclear-powered submarines.

Mr. Xi and Mr. Putin have denounced the initiatives. They have long seen those two main strategic prongs of Washington — promotion of democracy abroad and the deployment or sharing of troops and military equipment — as enormous threats to their nations.

“It is hoped that the U.S. side will take off their tinted glasses, discard the Cold War mind-set, view China-Russia relations and cooperation objectively, recognize the prevailing trend of the times and do more things that are beneficial to world peace and development,” Liu Pengyu, a spokesman for the Chinese Embassy in Washington, said when asked to comment for this article.

Alexander Gabuev, the chair of the Russia in the Asia-Pacific Program at the Carnegie Moscow Center, said that the joint statement from China and Russia was a notable public milestone, but that the most important cooperation was occurring beneath the surface. In particular, he said, weapons sales from Russia to the Chinese military should be of great concern to American policymakers.

Mr. Gabuev also noted that because the two nations settled territorial disputes along their 2,700-mile border in 2008 and have increased their military cooperation, Moscow felt confident enough to move troops from its east to near Ukraine to prepare for a potential invasion — drawing down Russian troops on the borders with China and Mongolia to their lowest level since 1922.

However, the two nations also compete and disagree on major issues. China has a growing footprint in Central Asia, whose former Soviet republics are viewed by Moscow as within its sphere of influence. China insists it is now a power in the Arctic, a region Mr. Putin has wanted to dominate. And the country has important trade relations with nations across the former Eastern European bloc.

China is Ukraine’s largest trade partner, and Beijing has acknowledged the nation’s sovereignty for decades. It has never recognized Russia’s 2014 annexation of Ukraine’s Crimean Peninsula. Notably, the joint statement made no explicit mention of Ukraine.

“When I was in the government, we would take a very hard look at China’s calculations and find those things that were not compatible with what Putin was trying to do and work on that basis,” said Daniel Russel, a former assistant secretary of state for East Asian and Pacific affairs. “The fact is that it’s so late in the game, and the three sides have moved so far in this unequal triangle, that it’s really not going to be easy to try to undo that.”

Eric Schmitt contributed reporting.