Russia’s huge military exercise along its western border this week has increased nervousness among neighboring countries as well as straining relations with NATO.

The “Zapad 2017” military exercise, running from the 14^th until the 20^th September, is held every four years and will on this occasion include activities on the borders of Poland, Lithuania, and Belarus.

The main purpose of Zapad, which literally means west, is to simulate defense and counter-attack in case of war with NATO member countries.

Security expert Otilia Dhand at Teneo Intelligence told CNBC Wednesday that while similar exercises have happened regularly since 1999, tension is enhanced this time because of recent history.

“Since the last Zapad exercises in 2013, Russia has annexed Crimea and fomented unrest in eastern Ukraine, where the conflict is ongoing.

“The Baltic members of NATO are therefore especially concerned over the possibility that these exercises may involve incursions into their territorial waters and airspace, as well as provide cover for building up Russian forces at bases close to their borders,” she said.

Since 2014, and the fall of Ukrainian President Viktor Yanukovych, Russian-backed soldiers have taken control of strategic positions within the Ukrainian territory of Crimea.

Dhand said that this week’s exercises could allow Moscow to funnel more equipment to separatist factions installed in eastern Ukraine.

She added the planned military build-up could also allow Russia to leave behind more forces and hardware at bases in Belarus and the satellite region of Kaliningrad.

NATO has said it has no plans to respond to the maneuvers but has expressed concern that Russia’s offer to allow three experts to attend some of the exercises falls well short of agreed international obligations.

“Briefings on the exercise scenario and progress; opportunities to talk to individual soldiers; and overflights over the exercise. This is something that is part of the Vienna document, an agreement regulating transparency and predictability relating to military exercises,” said NATO Secretary General Jens Stoltenberg in a speech made in Estonia on September 6.

Officially, the exercise involves just under 13,000 troops but Igor Sutyagin, senior research fellow of Russian Studies at the Royal United Services Institute, said in a note Tuesday that this number will be easily surpassed.

“Top of the concerns is the number of troops participating in the exercise.

“To avoid foreign monitoring, Russia and Belarus announced that just 12,700 troops will be participating in the exercise, below the Vienna Document threshold (of 13,000),” Sutyagin explained.

And the RUSI analyst said while Lithuania and Ukraine have put the estimated figure at around 140,000 troops, the true number may well sit around the middle.

“A forensic analysis of the data available about these exercises offers conclusive evidence that Russia is being economical with the truth.

“This analysis suggests the number of troops participating in the exercises will be closer to 65 to 70,000,” the security expert added.

Sutyagin arrived at this figure by identifying regiments openly participating in Zagad as well as noting a Russian Ministry of Defense request for 4,162 train carriages to transport troops and military hardware to and from Belarus.

“This represents a massive increase on the 2015 and 2016 transport requirement, which were 125 and 50 carriages respectively, a rise in requirements which one can only assume as necessary to fulfilling the day-to-day needs of the Russian forces permanently deployed to Belarus,” he said.

The Russian Ministry of Defense has posted details of the exercise on its website and described the Zapad 2017 anti-terror exercise as purely defensive.

“The exercise stipulates that some extremist groups have penetrated to the territories of the Republic of Belarus and the Kaliningrad region of the Russian Federation in order to carry out terrorist attacks and destabilize the Union State of Russia and Belarus.”

The official statement added that in addition to the claimed 12,700 troops, around 70 aircraft, 10 warships, and 250 tanks will take part in the Zapad exercise.