Russia declaring parts of Ukraine "independent" is a "blatant violation of [its] international commitments." So said White House Press Secretary Jen Psaki. She said President Biden will issue an executive order that doesn't allow Americans to invest, trade, or finance in certain parts of Ukraine. Anybody who does operate in those areas can be sanctioned.
These are separate and in addition to the economic measures prepared by the United States with its allies in case Russia further invades Ukraine. She explained other measures relating to Monday's Russian announcement will be announced as well. A White House official has also said that Biden spoke to Ukrainian President Zelensky for almost 40 minutes on Monday afternoon.