"NATO buildup on the Eastern flank, or in the so-called 'front-line countries,' is an unprecedentedly dangerous step that violates all agreements, including the Russia-NATO Founding Act, which is still in force," the ministry's spokesman Alexander Lukashevich said at a news briefing.
The Founding Act on Mutual Relations, Cooperation and Security between NATO and the Russian Federation was signed in Paris, France, in May 1997.
Lukashevich's words came after the announcement that NATO will deploy forces in Romania.
"If the Romanian government considers it acceptable to increase the presence of NATO groups on its land, as well as by creating such a powerful 'armed fist,' they just have to be aware of the responsibility and the consequences of such move," Lukashevich said.
In February, NATO said that its Response Force, stationed close to Russia in Eastern Europe, will more than double to include 30,000 military personnel.
The nearly year-long conflict in southeast Ukraine has served as a justification for NATO expansion in Eastern Europe. The military bloc, along with its political and economic counterparts, claims Moscow is involved in the ongoing fighting.
Russia has expressed concern over NATO expansion in the East, and over the buildup of NATO forces along its borders amid the Ukrainian conflict.