The United States on Thursday imposed sanctions on maritime companies and vessels for shipping oil sold above the G7’s price cap, as Washington seeks to close loopholes in the mechanism designed to punish Moscow for its war in Ukraine.

The U.S. Treasury Department in a statement said it slapped sanctions on three United Arab Emirates-based companies and three vessels owned by them in the action, accusing the vessels of engaging in the export of Russian crude oil priced above the $60 a barrel cap. It said the vessels used U.S.-person services while transporting the Russian-origin crude oil.

The U.S., other G7 countries and Australia imposed the cap last year, seeking to reduce Russia’s revenues from seaborne oil exports as part of sanctions for its invasion of Ukraine.

The cap bans Western companies from providing maritime services, including insurance, finance and shipping, for Russian seaborne oil exports sold above $60 a barrel, while seeking to keep oil flowing to markets. Caps also were imposed on Russian fuel exports.

“Shipping companies and vessels participating in the Russian oil trade while using Price Cap Coalition service providers should fully understand that we will hold them accountable for compliance,” Deputy Treasury Secretary Wally Adeyemo said in the statement.

“We are committed to maintaining market stability in spite of Russia’s war against Ukraine, while cutting into the profits the Kremlin is using to fund its illegal war and remaining unyielding in our pursuit of those facilitating evasion of the price cap.”

Thursday’s action freezes any U.S. assets of those targeted and generally bars Americans from dealing with them.