As Russian troops take part in what may be the largest military exercise since the Cold War, NATO has called on Moscow to be more “transparent” about its exercises.

The Zapad 2017 military exercise is held on the border with Poland, Lithuania and Belarus. The main purpose is to stimulate defense and counterattack in case of war with NATO member countries. The last time Russian President Vladimir Putin sent troops to Belarus a new war erupted.

Speaking exclusively to CNBC on Thursday, Jens Stoltenberg, Secretary General of NATO, called on Russia to be “transparent” about its military exercises to avoid any “misunderstandings or miscalculations.”

“Every nation, also Russia, has the right to exercise its forces but it’s important this is done in a transparent way to prevent misunderstandings, miscalculations,” he said.

“And we’ve seen before, Russia using military exercises as a disguise for aggressive actions against other countries; that was the case in 2008 and that was the case in 2014 when they annexed Crimea,” Stoltenberg said.

In 2014, Russia staged exercises near its border with Ukraine. After these military preparations many troops stayed in the area to participate in the subsequent war with Ukraine. Back in 2008, military exercises also preceded Russia’s invasion of Georgia.

Russia has said that some 12,700 troops will be involved but the actual figure is not certain. In 2013 for example, the Zapad exercises were publicly claimed to involve about 12,000 troops but NATO believes that number was more like 90,000.

“This is part of a pattern, which we’ve seen developed over several years with a more asserted Russia, a Russia which has tripled defense spending over the last years, which is using force against neighbors and which is exercising much more modern capabilities in a more aggressive way,” Stoltenberg said.

“The danger of incidents or accidents has increased because there are more troops, more military forces along our borders, there are more exercises, and that’s exactly why we are so focused on transparency, predictability,” NATO’s chief added.

Lithuania, which borders Belarus and formerly part of the Soviet bloc, is wary of the increased threat and unpredictability of Russia.

President of Lithuania Dalia Grybauskaite told CNBC Thursday that Russia has built-up its equipment and its cooperation with Belarus, which could pose a threat to Lithuania.

Grybauskaite believes that there’s a “very large” probability that part of Russia’s equipment, including troops, will be kept in Belarus after the military exercise.

“Russia is still very, very unpredictable, and it has proved this unpredictability with its activities in occupying Crimea, Ukraine, and Georgia,” she said.

NATO has increased its presence in the Baltic countries after Russia’s annexation of Crimea. About 1000 German troops are in Lithuania; U.S., British and Canadian-led units are also present in Poland, Estonia, and Latvia.

“We are prepared as never before to react, to watch, to see what’s happening behind our borders,” Grybauskaite told CNBC.

“History teaches us that we need to see and watch and prepare for the activities of Russia,” she added.