Senator Steve Daines was the first U.S. Senator to visit Ukraine since Russia’s invasion and he’s now calling for tougher sanctions on Russia’s financial institutions following his visit.

Senator Daines is pushing the US Treasury to tighten its sanctions on a range of companies, warning that gaps in the current regime are proving to strengthen the hand of Russian president Vladimir Putin.

“Following my visit to Ukraine last month, it is abundantly clear to me that the U.S. needs to take additional steps to stop Putin’s atrocious war crimes. I appreciate the Administration’s announcement of new sanctions on executives from Gazprombank and Sberbank, but believe further action is necessary. We must act to end this war expeditiously, and that means immediately ratcheting up sanctions on Russia and Vladimir Putin himself,” Daines wrote.

Daines also requested Yellen add Rosneft, Gazprom and Gazprombank to the Specially Designated Nationals And Blocked Persons List. It would put any company participating in business with them at risk of violating US sanctions.

This would make it much more difficult for European companies to buy Russian gas. American banks might be discouraged from participating in business.