NATO announced it will stand by Ukraine amid an unusual number of Russian troops massing near Ukraine’s borders, Jens Stoltenberg, NATO Secretary-General, announced Monday in Brussels.

“In recent weeks, we have seen large and unusual concentrations of Russian forces close to Ukraine’s borders,” Stoltenberg indicated during a press conference. “Any further provocation or aggressive actions by Russia would be of serious concern.”

Stoltenberg did not speculate about Russia’s intentions, which are unclear. However, Russia has taken similar actions in the past to threaten Ukraine.

Dmytro Kuleba, Minister of Foreign Affairs of Ukraine, believes that the military buildup is an attempt to destabilize Ukraine from the inside. Russia has been supporting separatist militants in Donbas, a region in southeastern Ukraine.

“The military build-up is the main plan, and it will be accompanied by efforts to destabilize Ukraine from the inside, or . . . the military build-up will serve as a background force, as a background argument for destabilizing efforts undertaken by Russia domestically,” Kuleba explained at the press conference.

Russia annexed Crimea from Ukraine after the Ukrainian Revolution in 2014, when Russia determined the results of the revolution to be an illegal coup of former president Viktor Yanukovych. Russia then invaded Crimea and declared its independence, making the nation and its city of Sevastopol federal subjects of Russia.

The Russo-Ukrainian conflict is still ongoing, fighting over regions like Crimea and Donbas. Many western nations and the U.N. consider this annexation a violation of international law. The action by Russia resulted in the country being suspended from the G8.

Stoltenberg insisted during the press conference that “NATO's presence in the Black Sea region, in the Baltic region is a defensive presence. We have increased our military presence in Eastern part of Alliance since 2014 as a result of Russia's aggressive actions against Ukraine.”