In preparation for an even larger drill in September, 12,000 Russian soldiers this week are conducting air-defense exercises in the Ural mountains and western Siberia, Reuters reported.

The war games, which began on Monday and are to last until Thursday, also involve 250 aircraft.

Russia has checked its combat readiness in some of the largest military drills ever since it annexed Ukraine's Crimea peninsula in March of last year, the newswire noted. This time around, its focus is on developing a command system in field conditions, as well as on setting up all-round aviation-security measures in new base areas, the Russian defense ministry noted.

The surprise exercises, which CNN called "a show of strength," may be a response to a long-planned European drill led by Norway, the news channel speculated. According to the Russian news agency Tass, forces planned to use 689 units of "various weapons and military equipment" and aim cruise missiles at an "imaginary enemy."

The North Atlantic Treaty Organization, meanwhile, expressed worry about Moscow's surprise war games.

"We are concerned by the size, scale and frequency of Russia's snap exercises," an unnamed NATO official told CNN. "Three of them have included over 80,000 troops; some of them have been used to deploy troops to annex Crimea, mass forces on Ukraine's border and support the separatists," the source added.

In Washington, President Barack Obama on Tuesday decried what he called Russia's "increasingly aggressive" posture with respect to Ukraine, Voice of America noted. Obama was hosting NATO Secretary-General Jens Stoltenberg, with whom he assessed the challenges the pact faces as it aims to protect alliance members.

Obama and Stoltenberg discussed the situation in Ukraine, including the steps the treaty organization has taken to ensure the security of frontline states, such as Ukraine, the semi-official broadcast service added.

"I was pleased to hear that not only the United States but also our other NATO allies have been doing what's necessary to make sure that we have positioned the resources and assets necessary to protect all NATO members," Obama said.