The source, speaking on condition of anonymity, said: "What we're seeing is non-lethal military assistance and economic support that stops short of wholesale sanctions evasion."

The source did not elaborate, and Reuters could not independently verify this account.

The United States has warned the Chinese government of consequences should China provide weaponry to Russia for use against Ukraine.

U.S. officials view the current activity as concerning and believe it is "a significantly scaled-down version of the PRC’s (Peoples Republic of China) initial plan, which was to sell lethal weapons systems for use on the battlefield," the source said.

It is unclear if the Chinese government is aware of the activity, the source said.

U.S. officials are reaching out to Chinese authorities through diplomatic channels, the source said.

"We will continue to communicate to China the implications of providing material support to Russia's war against Ukraine," said White House press secretary Karine Jean-Pierre.