On Sunday, the State Department issued a Security Alert urging American citizens to leave Russia by commercial means available:

An increasing number of airlines are cancelling flights into and out of Russia, and numerous countries have closed their airspace to Russian airlines. U.S. citizens should consider departing Russia immediately via commercial options still available. The U.S. Embassy reminds U.S. citizens that the Department of State’s Travel Advisory level for Russia is at “Level 4: Do Not Travel.”