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.
Russian officials recently denied Polish press reports that the Zapad-13 would include a notional nuclear attack on Warsaw. However, Russian officials have said the war games will involve practicing precision air and missile strikes.
On the Russian air base, Russian air force chief Lt. Gen. Vladimir Bondarev announced in June that the air base for Su-27 jets in Belarus would be opened near the city of Lida, near the border with Poland and Lithuania.
Bondarev said the warplanes would bolster a 1997 defense agreement between the two countries in response to NATO expansion.
It will be Russia’s first military base in Europe since the collapse of the Soviet Union, and along with it the Warsaw Pact, in 1991. Moscow currently has military bases in Armenia, Tajikistan, and Kyrgyzstan.
Belarus President Alexander Lukashenko has moved away from democracy and shifted the former Soviet republic away from Europe and toward Russia, restoring many symbols from Soviet Belarus.
In response to Polish press reports on the simulated nuclear strike on Poland’s capital, state-run Interfax news agency in April denied the reports. "Claims that West 2013 will allegedly practice a preventive nuclear strike on Warsaw are nothing but imagination of Polish journalists," a high-ranking officer was quoted as saying.
Belarusian Deputy Defense Minister Maj. Gen. Pyotr Tsikhanowski said in June, "The theme of the exercise is the training and the engagement of troops in order to ensure the military security of the Union State [of Belarus and Russia]." New weapons and military equipment will be tested during the exercises, he added.
War game scenarios include an "escalation of relations with countries based on interethnic, interreligious differences, and territorial claims," he said.
"At the same time, the conflicting states are hypothetically located within the actual borders of Belarus and the three western and northwestern regions of Russia," Tsikhanowski said.
U.S. military officials said the war games are part of a larger Russian effort to use military power to bolster its position in the former Soviet republics with a larger military presence.
Russia also has a ballistic missile warning radar at Baranovichi, Belarus, and a Navy communications facility used to communicate with Russian submarines. Moscow also supplied Belarus with advanced S-300 missile defenses and Tor-M2 surface-to-air missiles.
The exercises drew criticism from Eastern European NATO governments.
"Russia has officially stated that these are anti-terrorism exercises," Lithuanian Defense Minister Juozas Olekas told AFP. "But the number of participants and amount of military equipment indicates that that this is not their agenda."
Senior Estonian military official Lt. Col. Eero Rebo said: "The Kremlin claims that the exercise is about fighting terrorism, but based on the information we have on Zapad 2013, the exercise has an anti-West agenda."
"If you look at the Baltic sea region, the strategic balance has been changing quite drastically in the last decade, and not in our favor," Latvian Defense Minister Artis Pabriks said Friday.
"We are concerned because we see such large-scale exercises in context," he added. NATO approved special defense plans for Lithuania, Latvia, and Estonia, which have a combined population of just over six million, in 2010.
NATO will hold an Eastern European exercise in November called Steadfast Jazz in the Baltic region and Poland with around 6,000 NATO troops.
The alliance claims the exercises from Nov. 2 to Nov. 9 are not aimed at Russia.
"For the past 10 to 12 years we have become incredibly proficient at the counter-insurgency mission that we have been fighting in Afghanistan," NATO's military commander, U.S. Air Force Gen. Philip Breedlove, told reporters in Brussels Sept. 18.
To defend an alliance state, "we have to be prepared for the more high end of military operations," he said, according to Reuters.
The exercise scenario will involve NATO forces ousting a foreign invader that Breedlove denied was a hypothetical counterstrike to a Russian invasion.
Meanwhile in China, Chinese military forces are engaged in large-scale exercises involving some 40,000 troops.
The People's Liberation Army announced its "Mission Action 2013" began Tuesday.
Troops from the Nanjing and Guangzhou military regions and air forces were set to take part.
The PLA troops will conduct maneuvers over large areas using military ground vehicles, trains, ships, and aircraft to test forces for "real war" conditions.
The exercises will include coordination between military and civilian assets, including civilian jets and trains to transport forces, according to state-run Chinese press reports.
Chinese aircraft taking part in the war games include jet fighters, bombers, and other aircraft.
The exercise also will seek to boost Chinese combined arms warfare capabilities that integrate air, naval, and ground forces for long-distance fighting, joint air defense, and joint warfighting on unfamiliar terrain.