The NATO alliance held joint military exercises Monday in Poland, Lithuania, Estonia and Latvia amid escalating tensions between eastern European nations and Russia. NATO has stepped up the size and training of its forces in recent months in response to a marked uptick in Russian military activity, including Moscow’s purported involvement in the eastern Ukraine conflict.

Dubbed “Saber Strike 2015,” the annual NATO exercises will involve 13 member nations and more than 6,000 soldiers, Agence France-Presse reported. United States military personnel will spearhead the drills, which were slated to continue through June 19.  

“This is one of the biggest exercises in Lithuania since we joined NATO [in 2004],” Lithuania military commander Almantas Leika said, as quoted by AFP. “The huge allied presence demonstrates solidarity with the countries of this region.”

Several Eastern European nations, including some that are not NATO members, have expressed increasing concern with the rise in Russia’s military exercises this year. The Kremlin has ordered a series of snap-readiness drills for its land, sea and air forces in areas as far away as the Arctic Ocean. Latvia, Finland and Sweden have each reported encounters near their territory with suspected Russian submarines.

Representatives from Latvia, Lithuania and Estonia banded together in May to request a permanent detachment of NATO troops within their borders to serve as a counterweight to Russia’s military, Reuters reported. NATO has defended the three nations’ air space since 2004, but has yet to address the request. The alliance also announced the expansion of its rapid reaction force earlier this year to consist of 30,000 troops.

Russia drew international criticism for its March 2014 annexation of Crimea and for its alleged support for pro-Moscow separatist rebels active in eastern Ukraine. A United Nations report released Monday said more than 6,400 people have died in the eastern Ukraine conflict and found evidence, but not definitive proof, of the Russian military’s involvement in hostilities. The Kremlin has denied culpability and claims any Russian nationals active in Ukraine are volunteers.