Xi Jinping hailed the "special" and "unbreakable" relationship between China and Russia in a telephone call with his counterpart Vladimir Putin on Monday, China's state-owned media reported.
The two heads of state praised bilateral ties, which are at their "highest level in history," said an announcement by Russia's presidential office after the conversation.
The Chinese president called for further "strategic cooperation" between the two nations, which have been sharing growing economic and military ties amid Western sanctions against both Beijing and Moscow.
The "special advantages of the China-Russia relationship" were most obvious in times of crisis, China's Xinhua News Agency quoted Xi as saying. A high level of mutual trust and friendship was evidenced by the support shown for each other's core interests, he added.
Xi said the two countries share a relationship that is "unbreakable under winds of change" elsewhere in the world and not affected by external factors. By strengthening their strategic partnership, China and Russia will be capable of withstanding any attempts to "suppress and divide" them, he stressed.
Last week, Chinese and Russian military warplanes conducted a joint patrol over the Sea of Japan and the East China Sea in a sign of strengthening ties between Beijing and Moscow as the United States undergoes a presidential transition.
Japan and South Korea both scrambled fighter jets to monitor the contingent of four Chinese H-6K bombers and two Russian Tu-95 bombers on December 22, said reports from Tokyo and Seoul.
On the same day, foreign ministers Wang Yi of China and Sergei Lavrov of Russia held a call to round up a year of "satisfying" bilateral cooperation. They each criticised Washington for "suppressing" Beijing and Moscow with unilateral sanctions, a Xinhua read-out said.
China and Russia have expressed a willingness to work with President-elect Joe Biden, but both governments have suggested the onus is on the next U.S. administration to first change tack.
Meanwhile, Biden, who called Xi a "thug" during his election campaign, reiterated on Monday his commitment to stand up to China by working with other democracies in the Asia-Pacific.
"As we compete with China and hold China's government accountable for its abuses on trade, technology, human rights and other fronts, our position will be much stronger when we build coalitions of like-minded partners and allies," the president-elect said in remarks following a briefing by national security and foreign policy advisers.
From his transition headquarters in Wilmington, Delaware, he added: "On any issue that matters to the U.S.-China relationship, from pursuing a foreign policy for the middle class—including a trade and economic agenda that protects American workers, our intellectual property and the environment, to ensuring security and prosperity in the Indo-Pacific region, to championing human rights—we are stronger and more effective when we are flanked by nations that share our vision."
The Biden administration will recognize China and Russia as the greatest threats to U.S. national security and influence in Asia, but America under his leadership will face a continental bond between Beijing and Moscow that is tightening as mutual grievances against Washington grow.
The neighbors are on course to deepen their strategic cooperation next year, which will mark the 20th anniversary of the 2001 Sino-Russian Treaty of Friendship signed by Putin and then Chinese President Jiang Zemin.
In October, before the U.S. presidential election, Putin hinted that a Russia-China military alliance, once thought highly unlikely, was "possible to imagine" in the future.