Russia has asked China for military ​equipment to ​support its invasion of Ukraine, ​sparking concern in ​the White House that Beijing ​may undermine western efforts to help Ukrainian forces defend their country.

US officials told the Financial Times that Russia had requested military equipment and other assistance since the start of the invasion. They declined to give details about what materiel Russia had requested.

Another person familiar with the situation said the US was preparing to warn allies about the situation amid some indications that China may be preparing to help Russia. Other US officials have also said there were signs that Russia was running out of some kinds of weaponry as the war in Ukraine approaches the start of its third week.

The White House did not comment. The Chinese embassy in Washington did not respond to a request for a comment.

The revelation comes as Jake Sullivan, US national security adviser, heads to Rome for talks on Monday with Yang Jiechi, China’s top foreign policy official. People familiar with the situation said Sullivan would sternly warn China that any efforts to help Russia prosecute the invasion or avoid western sanctions would have consequences.

Before leaving Washington on Sunday, Sullivan warned China not to try to “bail out” Russia by helping Moscow to circumvent the sanctions that the US and its allies have imposed on Putin and his regime.

“We will ensure that neither China, nor anyone else, can compensate Russia for these losses,” Sullivan told NBC television on Sunday. “In terms of the specific means of doing that, again, I’m not going to lay all of that out in public, but we will communicate that privately to China, as we have already done and will continue to do.”

The request for equipment and other kinds of unspecified military assistance comes as the Russian military struggles to make as much progress in Ukraine as western intelligence believe they expected.

It also raises fresh questions about the China-Russia relationship, which has grown increasingly strong as both countries express their opposition to the US over everything from Nato to sanctions.

China has portrayed itself as a neutral actor in the Ukraine crisis and has refused to condemn Russia for invading the country. The US has also seen no sign that Chinese president Xi Jinping is willing to put any pressure on Vladimir Putin, his Russian counterpart.

The two leaders signed a joint statement in Beijing last month describing the Beijing-Moscow partnership as having “no limits”, in another sign that the two capitals were drawing even closer together.