According to U.S. officials, Russia asked China to give it military equipment and support for the war in Ukraine after President Vladimir V. Putin began a full-scale invasion last month.

An official said that Russia has asked China for more economic help to counteract the effects of sanctions imposed by the United States and European nations.

American officials are determined to keep their means of collecting the intelligence on Russia secret. The officials wouldn't discuss China's reaction to the requests.

China's president has strengthened his relationship with Mr. Putin and has stood by him as Russia has killed hundreds or thousands of people in its military campaign in Ukraine. American officials are watching to see if China will act on Russian requests. Jake Sullivan, the White House national security adviser, is going to meet with a member of the Chinese Communist Party in Rome on Monday.

Mr. Sullivan will warn Mr. Yang about any future Chinese attempts to bolster Russia in its war or undermine the United States.

Mr. Sullivan said on CNN that there would be consequences for large-scale sanctions evasion efforts or support to Russia.

He said that they would not allow that to happen and that there would be a "lifeline to Russia" from the economic sanctions from anywhere in the world.

Sullivan did not make any mention of military support from China, but other U.S. officials spoke about the request from Russia on the condition of anonymity because of the sensitivity of diplomatic and intelligence matters.

The spokesman for the Chinese Embassy in Washington said he had never heard of the request from Russia.

The consequences of China aligning with Russia will be laid out by the Biden administration if China continues to support Russia. Some U.S. officials think it's possible to discourage Beijing from helping Moscow. U.S. officials say that Chinese leaders may be content to offer rhetorical support for Moscow, but may not want to provide military support for the war.

Mr. Sullivan said that the Chinese might not have known the full extent of the Russian leader's plans before the invasion.

The two share a desire to weaken American power and have met 38 times, more than any other head of state.

China has traditionally bought military equipment from Russia. Russia has increased its sales of weaponry to China. Russia could use missiles and drones from China in its campaign.

There has been some evidence that Russian missile supplies have been running low, according to independent analysts.

The White House criticized China last week for helping spread misinformation about the United States. In recent days, Chinese diplomats, state media organizations and government agencies have used a range of platforms and official social media accounts to amplify a conspiracy theory that says the Pentagon has been financing biological and chemical weapons labs in Ukraine. The theory has been promoted by right-wing political figures.

The Chinese ambassador to the U.N. supported his Russian counterpart at the UN Security Council meeting on Friday.

We should all be on the lookout for Russia to use chemical or biological weapons in Ukraine, or to create a false flag operation using them, now that Russia has made these false claims.

China is involved in the Iran nuclear negotiations, which have stopped because of new demands from Russia on relief from the sanctions imposed by Western nations.

American officials are trying to figure out how much China would support Russia in those talks. Officials from the nations involved had been close to agreeing on a new version of the Obama-era nuclear limits agreement, which President Donald J. Trump withdrew from. Mr. Sullivan and Mr. Yang might discuss Iran on Monday.

Current and former U.S. officials say the Rome meeting is important because of the stakes in the Ukraine war and the possibility of Russia and China joining forces against the United States.

Evan Medeiros was a senior Asia director on the National Security Council during the Obama administration.

He said that the U.S. would probably lay out the costs and consequences of China's involvement in Russia's invasion.

The U.S. is trying to get Mr. Putin to distance himself from Mr. Xi. Others see Mr. Xi as a lost cause and prefer to treat China and Russia as committed partners, hoping that will galvanise policies and coordination among Asian and European allies to contain them both.

Chinese officials have consistently voiced sympathy for Russia during the war in Ukraine. Even as they express concern for the humanitarian crisis, they have refrained from mentioning a Russian invasion.

The phrase "sovereignty and territorial integrity" is a common phrase in Chinese diplomacy, but they don't say which nation they support.

China and Russia issued a 5,000-word statement on Feb. 4 saying their partnership had no limits. Senior Chinese officials asked senior Russian officials not to invade Ukraine before the end of the Games, according to U.S. and European officials.

American officials held secret talks with Chinese officials to discuss intelligence showing that Mr. Putin was planning to launch a war. The Chinese officials were against the Americans at every meeting.

William J. Burns, the C.I.A. director, said in a Senate hearing that he believed Mr.

On Tuesday, the leader of China repeated his country's talking points on the war in a video call with the leaders of France and Germany. He said that all nations should show maximum restraint, and that China was very upset by the war on the European continent. He didn't say that Russia had started the fighting.

Large Chinese companies will most likely refrain from violating sanctions on Russia for fear of damaging their global commerce. On Thursday, some Russian news articles and commentary questioned China's commitment to Russia after news agencies reported that China was refusing to send aircraft parts to the country.

U.S. officials often remind the public that Russia has few friends. Russia's outreach to its partners is a sign of the difficulties it is having in trying to subdue Ukraine, according to officials.

The United States and Europe have put more pressure on Moscow.

Russia got assistance from Belarus in the build up to war. Moscow has been trying to evade sanctions. The European Union imposed sanctions on the country. The penalties limit the flow of money into the country from Europe and block some banks from using the financial messaging system.

The U.S. ambassador to the Organization for Security and Cooperation in Europe accused the country of being a "co-aggressor" and having "stabbed your neighbor in the back."

President Lukashenko said his military wouldn't join the war. Some injured Russian soldiers were evacuated to hospitals in that country after Russia launched missiles there.

President Bashar al-Assad of Syria, who owes his government's survival to Russian intervention in the Syrian civil war, also declared support for Moscow's invasion. According to the Pentagon, Russia tried to recruit Syrians to join the war.

While there are no details of how many recruits Moscow has enlisted or if they have arrived in Ukraine, American officials said Russia's efforts were an indicator of the strategic and tactical problems that have plagued its commanders.

European officials said that Russian military contractors with experience fighting in Syria and Libya secretly entered eastern Ukraine to lay the groundwork for the invasion.