The United States will not make an exception for American companies, including Exxon Mobil, seeking to drill for oil in areas prohibited by U.S. sanctions on Russia, Treasury Secretary Steven Mnuchin said on Friday.

The unusually direct statement served to clarify that the United States would maintain a tough stance on sanctions against Moscow.

"In consultation with President Donald J. Trump, the Treasury Department will not be issuing waivers to U.S. companies, including Exxon, authorizing drilling prohibited by current Russian sanctions," Mnuchin said in a statement.