Russian Foreign Minister Sergey Lavrov on Thursday accused the U.S. and NATO of risking a “nightmare scenario of military confrontation” in Europe, warning the alliance against expanding military infrastructure in Ukraine.

“The alliance’s military infrastructure is being irresponsibly brought closer to Russia’s borders in Romania and Poland, deploying an anti-missile defence system that can be used as a strike complex,” Lavrov said in remarks at a conference of the Organisation for Security and Co-operation in Europe (OSCE), per BBC News.

The top Russian diplomat’s comments came amid fears that Moscow is planning an invasion of Ukraine. Tens of thousands of Russian troops have gathered along Ukraine’s border.

Since Russia’s unilateral annexation of Crimea in 2014, Ukraine has emerged as a major geopolitical dividing line between the West and Russia. That same year, a war began between Kremlin-backed rebels and Ukrainian troops in the eastern Donbass region. The conflict, which Russia claims it’s not involved in, has killed over 13,000 people.

Ukraine is not a full NATO member, but has sought to join the alliance for years and maintains a robust partnership with it. Members of the security alliance have given military aid to Ukraine, and NATO and Ukrainian troops have participated in joint exercises.

Russian President Vladimir Putin views the alliance’s growing influence in the former Soviet republic as an existential threat. Putin has expressed particular dismay over NATO and U.S. military activities in the Black Sea region.

“The threat on our western borders is, indeed, rising, as we have said multiple times,” Putin said at a ceremony for ambassadors at the Kremlin on Wednesday, The New York Times reported. “In our dialogue with the U.S. and its allies, we will insist on developing concrete agreements prohibiting any further eastward expansion of NATO and the placement there of weapons systems in the immediate vicinity of Russian territory.”

Putin said Russia needed “legal guarantees” that Moscow’s security concerns in the region will be respected.