By David Lawder and Ann Saphir

SAN FRANCISCO U.S. Treasury Secretary Janet Yellen on Friday said the U.S. government had seen evidence that Chinese firms may be aiding in the flow of equipment to Russia's war effort despite Western sanctions, and said she had urged China to crack down.

Yellen said she raised the issue during two days of meetings with Chinese Vice Premier He Lifeng, expressing concern that equipment "helpful to Russia's military" was evading sanctions and getting to Moscow to aid its war against Ukraine.

"I stressed that companies must not provide material support to Russia's defense industrial sector and that they will face significant consequences if they do," Yellen told reporters at a news conference in San Francisco.

"We are determined to do all that we can to stem this flow of material that aids Russia in conducting this brutal and illegal war," Yellen said, warning that any companies aiding Moscow's war effort could face sanctions.

She said the U.S. government had already imposed sanctions against a number of private firms, including some in China, that were helping Russia get equipment, along with some financial institutions that could be aiding that effort.

"We would like to see China crack down on this, especially when we're able to provide information," she said. She gave no further details on the names of the companies involved or He's reaction.

Yellen stressed the Chinese firms in question were private and said she was not suggesting that this was occurring with knowledge of the Chinese government.

(Reporting by David Lawder and Ann Saphir, writing by Andrea Shalal and Alistair Bell)