Russian President Vladimir Putin held military drills near the borders of Lithuania and Poland earlier this month. The exercises featuring 9,000 soldiers and more than 55 naval ships were aimed at testing the military's combat readiness in Kaliningrad, which borders both of the NATO-member states.

"The exercise involved units of missile troops, artillery, mechanized infantry, airborne troops, marine forces, units of reconnaissance, communications, logistics, as well as combat and military transport aircraft, helicopters and ships of the Baltic Fleet," TASS news agency said, citing the Armed Forces General Staff.

Lithuania put its military units on high alert on Dec. 8 in response to the military exercises. Russia and NATO have been pointing fingers for months, with both sides accusing the other of building up its military forces in response to Putin's alleged support of rebel militants in Ukraine. NATO recently complained of an increase in Russian military flights near the borders of its member states.

“We certainly do not welcome that NATO intensifies” its presence “near our borders,” Russian Deputy Foreign Minister Sergei Ryabkov said last month. “We closely monitor everything that happens. We hope that NATO colleagues have the same attitude to what we do.”

Sweden and Denmark summoned Russia’s ambassadors this week after a passenger plane leaving Copenhagen on Dec. 12 was forced to change direction to avoid crashing into a Russian military jet. Russian Defense Ministry spokesman Igor Konashenkov said the jet hadn't broken international airspace rules or endangered civilian aircraft. 

U.S. Air Force Gen. Philip Breedlove, NATO's Supreme Allied Commander Europe, said the recent Russian flights were "of a different nature than we've seen in a while." "For the past 19 years, we have been trying to treat Russia as a partner, trying to bring the nations of Europe back together and now what we see is a very different kind of scenario," Breedlove told NBC News.