Mr. Xi’s endorsement of Russia’s opposition to any expansion by the North Atlantic Treaty Organization—a central demand from Moscow in its standoff with the U.S.-led allies over Ukraine—marked China’s most explicit support to date of the Kremlin in a confrontation.

Beijing has been careful not to green light Russia’s possible invasion of Ukraine. But the joint statement by Messrs. Xi and Putin on Feb. 4 nonetheless represented Beijing’s closest alignment with Moscow since the early years of the Communist bloc’s Cold War with the West. That has stirred up some unease in China’s official circles, according to people close to the government, because it signaled such a fundamental shift in China’s foreign policy.

“It’s one thing for China to back Russia in opposing NATO enlargement, as it costs nothing,” said Sergey Radchenko, a professor of international relations at Johns Hopkins University in Bologna. “It’s quite another for China to help Russia evade the economic sanctions it would face if it invades Ukraine.”