Treasury Secretary Janet Yellen warned China of “serious consequences” if it helps Russia evade U.S. sanctions, calling for even tighter economic restrictions on Moscow as the Ukraine war nears its one year mark.

“We have made clear that providing material support to Russia or assistance with any kind of systemic sanctions evasion would be a very serious concern for us,” Yellen said at the Group of 20 meeting in India on Thursday. “We will certainly continue to make clear to the Chinese government and the companies and banks in their jurisdiction about what the rules are regarding our sanctions and the serious consequences they would face for violating them.”

Yellen said that existing U.S. sanctions on Russia were working, painting a bleak picture of the Russian economy and the Russian defense industry, which she said has faced production halts. Still, she urged for more severe restrictions.

“We are seeking to strengthen sanctions and make sure that we address violations of sanctions,” Yellen said. “We have deprived them of many channels they can obtain material that is critical for their defense efforts. That is a particular focus we have in mind in respect to enforcement of existing sanctions and perhaps with respect to further sanctions.”

Yellen declined to specify U.S. intelligence on China’s efforts to help Russia evade American sanctions.

The warning to China comes as tensions between the U.S. and both China and Russia have flared in recent weeks. U.S. officials have quarreled with China after a suspected Chinese spy balloon traveled over the U.S. and was shot down earlier this month and the Biden administration has sharply criticized the Russian decision to pull out of a nuclear restriction between the two countries.

And as fighting continues in Ukraine, Yellen said she expects the U.S. to offer another $10 billion in support to Ukraine in the coming months. 

“We will stand with Ukraine in its fight — for as long as it takes,” Yellen said.