“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,” the State Department said in a statement.

It said more broadly that U.S. citizens should not travel to Russia and that anyone there now “should depart Russia immediately while limited commercial travel options remain.”

The State Department said flights out of Russia “are extremely limited at present and are often unavailable on short notice. Overland routes by car and bus are still open.”

But it said U.S. travelers in Russia or Americans living there who are planning to leave “should make independent arrangements as soon as possible. The U.S. Embassy has severe limitations on its ability to assist U.S. citizens, and conditions, including transportation options, may suddenly become even more limited.”

In addition, the State Department warned Americans “that the right to peaceful assembly and freedom of expression are not guaranteed in Russia. Avoid all political or social protests and do not photograph security personnel at these events. Russian authorities have arrested U.S. citizens who have participated in demonstrations.”