Russia began major joint military exercises with Belarus along the European Union's eastern flank, a show of strength that has rattled nervous NATO members.

Named Zapad-2017 (West-2017), the maneuvers, scheduled to last until September 20, are taking place on the territory of Moscow's closest ally Belarus, in Russia's European exclave of Kaliningrad and in its frontier Pskov and Leningrad regions.

Moscow said the drills involves 12,700 troops, 70 aircraft, 250 tanks and 10 battleships testing their firepower against an imaginary foe close to borders with Poland and the Baltic States.

In a statement announcing the start of the exercises Russia's defense ministry insisted the maneuvers are "of a strictly defensive nature and are not directed against any other state or group of countries." But NATO claims Russia has kept it in the dark and seems to be massively underreporting the scale of the exercises, which some of the alliance's eastern members insist could see more than 100,000 servicemen take part.

Meanwhile, four Chinese navy ships have departed for joint drills with Russia in the latest sign of growing cooperation between the two militaries that could challenge the U.S. armed forces' role in the Asia-Pacific. A destroyer, missile frigate, supply ship and submarine rescue ship departed Wednesday from the port of Qingdao, home to China's north sea fleet, the official Xinhua News Agency reported. The drills are being held in the Sea of Japan near the Korean Peninsula and the Sea of Okhotsk off the northern Japanese island of Hokkaido, Xinhua said. The exercises are the second stage of an annual joint drill, the first part of which was held July 22-27 in the Baltic Sea, the first time the countries had exercised together in the northern European waterbody.

July's joint drills in the Baltic stirred concern among countries in the region, where tensions are already high over increased displays of military force by both Moscow and NATO. Both Russia and China say the exercises are not directed at any third parties.

The war games come with tensions between Russia and NATO at their highest since the Cold War due to the Kremlin's meddling in Ukraine and the U.S.-led alliance bolstering its forces in Eastern Europe.

Moscow has held a stream of exercises since ties with the West plunged in 2014 over Ukraine, with the military claiming some drills included nearly 100,000 troops.

The Kremlin has vigorously defended its right to hold exercises and has long blamed the United States for ratcheting up tensions by expanding NATO up to its borders and holding its own provocative drills.

The Russian war games come as Ukraine on Monday launched annual joint military exercises with the U.S. and a host of other NATO countries. Meanwhile non-aligned Sweden has mobilized 19,000 soldiers for its biggest drills in 20 years which also include units from across Scandinavia and the U.S.