The meeting took place in Rome as Washington told allies in NATO and several Asian countries that China had signaled its willingness to provide military and economic aid to Russia to support its war, two U.S. officials said.

The U.S. message, sent in a diplomatic cable, also noted China was expected to deny those plans, said one of the officials, speaking on condition of anonymity.

"It's real, it's consequential, and it's really alarming," the second U.S. official said, although the U.S. government offered no public evidence to back its assertions of China's willingness to provide such aid to Russia.

After talks ended, the White House issued a short statement, saying Sullivan raised a "range of issues in U.S.-China relations, with substantial discussion of Russia's war against Ukraine."

"We have deep concerns about China's alignment with Russia at this time, and the national security adviser was direct about those concerns and the potential implications and consequences of certain actions," a senior administration official told reporters.

Sullivan described to Yang "the unity of the United States and its allies and partners ... in bringing costs on Russia for its actions," this official added.

The official described the meeting as "intense," reflecting "the gravity of the moment," although it had long been planned, was not timed to events in Ukraine and covered other issues including North Korea, Taiwan and tense bilateral relations.

The official said the exchanges had been "candid" but led to no specific outcomes.

Before the talks, U.S. officials had said Sullivan planned to warn of the isolation China could face globally if it supported Russia.

Officials of the United States and other countries have sought to emphasize in recent weeks that siding with Russia could carry consequences for trade flows, development of new technologies and expose China to secondary sanctions.

Chinese companies defying U.S. restrictions on exports to Russia may be cut off from American equipment and software they need to make their products, U.S. Commerce Secretary Gina Raimondo said last week.

"We have communicated very clearly to Beijing that we will not stand by...(and) we will not allow any country to compensate Russia for its losses," State Department spokesman Ned Price told a regular briefing in Washington.

It was Sullivan's first-known meeting with Yang since closed-door sessions in Zurich in October that sought to calm tensions after an acrimonious public exchange between the two in Alaska a year ago.

AVOID CONFLICT, CHINA SAYS

China's official Xinhua news agency cited Yang as saying that Beijing was committed to promoting negotiations to resolve the Ukraine conflict.

"China firmly opposes any words and deeds that spread false information and distort and smear China's position," Yang said, in an apparent oblique reference to Washington's claims about support for Russia. China and the United States should strengthen dialogue, properly manage differences, and avoid conflict and confrontation, he said.

Ryan Hass of the Brookings Institution said Chinese support for Russia "would considerably narrow its path for preserving non-hostile relations with the United States and the West" and not likely alter the trajectory of the conflict.

China is the world's-largest exporter, the European Union's largest trading partner and the top foreign supplier of goods to the United States. Any pressure on Chinese trade could have economic effects on the United States and its allies.

U.S. officials told Reuters on Sunday that Russia had asked China for military equipment after its invasion. Russia denied asking China for military assistance and said it has sufficient military clout to fulfill all of its aims in Ukraine.

Sino-U.S. ties, already at their lowest point in decades, took a further plunge last month when leaders Xi Jinping of China and President Vladimir Putin of Russia announced an upgraded "no limits" strategic partnership just weeks before the Ukraine invasion.

China, a key trading partner of Russia, has refused to call Moscow's actions an invasion, although Xi last week did call for "maximum restraint" and express concern about the impact of Western sanctions on the global economy, amid growing signs that they limit China's ability to buy Russian oil.

Russia itself calls its moves into Ukraine a "special military operation."

The United States and its allies have imposed sweeping sanctions on Russia and banned its energy imports, while providing billions of dollars of military and humanitarian assistance to Ukraine.