Pentagon intelligence agencies are closely watching Russian and Chinese war games now taking place in Europe and Asia involving tens of thousands of troops.
Meanwhile, NATO military forces are set to conduct large-scale maneuvers in November that will be designed to counter growing concerns of a westward Russian military encroachment, according to U.S. officials.
"The Russians are moving forces closer to Europe, and that is troubling," said a military official.
Russia's Zapad-13 military exercises in Belarus are scheduled to end Thursday. They included practice attacks on a western state, said one official familiar with reports of the maneuvers.
Some 13,000 Russian and Belarusian troops took part with over 60 aircraft and helicopters and up to 250 vehicles.
The forces practiced "rapid reaction" drills.