"On September 21, the Russian government began a mobilization of its citizens to the armed forces in support of its invasion of Ukraine," the embassy said in a statement posted on its website on Tuesday.
It warned that Russia "may refuse to acknowledge dual nationals’ U.S. citizenship, deny their access to U.S. consular assistance, prevent their departure from Russia, and conscript dual nationals for military service."
"Commercial flight options are extremely limited at present and are often unavailable on short notice," the statement said, adding that "overland routes by car and bus are still open."
US citizens "should not travel to Russia and those residing or travelling in Russia should depart Russia immediately while limited commercial travel options remain," according to the statement.
Source: PAP, ru.usembassy.gov