The move is the latest in a series of multinational training and security cooperation activities along the alliance's eastern front. It comes at a tenuous time in the relationship between the Kremlin and the fledgling administration of President Donald Trump, who just last week voiced unusually strong support for NATO after repeatedly objecting to its relevancy and funding. One hundred twenty troops from the 3rd Armored Brigade Combat Team, 4th Infantry Division, from Fort Carson, Colorado, arrived Wednesday at the Novo Selo training range in eastern Bulgaria, according to the Bulgarian statement. They're due to be joined by heavy military equipment and armored vehicles by the end of the week.

"This year joint exercises and training will be increased at the Novo Selo training grounds," said the ministry, adding that its goal is to develop and strengthen cooperation on a national and allied level.

Moscow has repeatedly criticized the deployments, calling them provocative and a threat to Russian security. Russian President Vladimir Putin reiterated that position Thursday at the annual board meeting of the Federal Security Service, a Russian agency that oversees national security and counterterrorism.

"At the NATO summit last July in Warsaw, Russia was declared the main threat to the alliance for the first time since 1989, and NATO officially proclaimed containing Russia its new mission. It is with this aim that NATO continues its expansion," Putin said.

"They are provoking us constantly and are trying to draw us into confrontation," he said.

"We are not in a position right now to collaborate on the military level, but our political leaders will engage and try to find common ground," he said.

"As we search for new common ground, we expect Russia to honor its commitment to the Minsk Agreements and work to deescalate the violence in the Ukraine," Tillerson said.

Russia's annexation of Crimea and ongoing tensions between pro-Russian separatists and government forces in eastern Ukraine have caused growing unease among NATO members in Eastern Europe and the Baltic states, presenting one of the biggest challenges for the alliance in its nearly 70-year history.

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US troops deploy to Bulgaria as part of NATO operation to support Eastern European allies - CNN