The United States will never recognize Ukrainian territory as anything other than part of Ukraine. Russia's referenda are a sham, a false pretext to try to annex parts of Ukraine by force in flagrant violation of international law, including the United Nations Charter. We will work with our allies and partners to impose additional swift and severe economic costs on Russia. The United States stands with our partners around the world—and with every nation that respects the core tenets of the U.N. charter—in rejecting whatever fabricated outcomes Russia will announce. We will continue to support the Ukrainian people and provide them with security assistance to help them defend themselves as they courageously resist Russia's invasion.
Joseph R. Biden, Jr., Statement on Russia's Annexation Referendums in Ukraine Online by Gerhard Peters and John T. Woolley, The American Presidency Project https://www.presidency.ucsb.edu/node/358061