U.S. President Barack Obama was due to depart for Europe on a key visit that takes him to the Baltic country of Estonia and then to Britain for a NATO summit.
Obama's visit comes as tensions between Russia and the West have reached levels not seen in decades.
The United States and its European allies have imposed a series of sanctions on Russia for the Kremlin's actions in eastern Ukraine where pro-Russian separatists have been battling Ukrainian government forces for months.
Obama is expected to arrive in Estonia during the evening of September 2.
The U.S. president will meet with Estonian officials on September 3, including President Toomas Hendrik Ilves, and then later that day Obama and Ilves will be joined by the presidents of the other Baltic states -- Latvia and Lithuania.
The NATO summit in Wales takes place on September 4 and 5.