ROME/WASHINGTON—US national security adviser Jake Sullivan on Monday raised concerns with Chinese diplomat Yang Jiechi about China’s alignment with Russia, as Washington warned of the economic penalties and global isolation that Beijing would face if it helps Moscow in its invasion of Ukraine.

The lengthy meeting took place in Rome, as the United States 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, 2 US officials said.

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

"It's real, it's consequential, and it's really alarming," the second US official said.

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

"They also underscored the importance of maintaining open lines of communication between the United States and China," the statement said.

Sullivan had planned to warn of the isolation China could face globally if it continued to support Russia, another US official said before the talks, without providing details.

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

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.

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 cause economic effects for the United States and its allies.

U.S. officials told Reuters on Sunday that Russia had asked China for military equipment after its invasion, sparking concern within President Joe Biden's administration that Beijing might undermine Western efforts to aid Ukraine by helping to strengthen Moscow's military.

Russia denied it had asked China's for military assistance and said it has sufficient military clout to fulfill all of its aims in Ukraine.

U.S.-Chinese ties, already at their lowest 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.

On Monday, Chinese foreign ministry spokesperson Zhao Lijian described as U.S. "disinformation" the reports that Russia was seeking military equipment from China.

Daniel Russel, who served as the top U.S. diplomat for East Asia under President Barack Obama and has close ties to the Biden administration, called the prospect of China serving as a mediator to end the war "far-fetched" even if "Beijing may talk a good game about ceasefires and mediation to insulate itself from blame."

(Reporting by David Brunnstrom, Michael Martina, Andrea Shalal and Phil Stewart in Washington and Antonio Denti in Rome; Additional reporting by Eduardo Baptista in Beijing; Editing by Heather Timmons, Will Dunham and Raissa Kasolowsky)