The NATO military alliance may increase its presence on the eastern borders of Europe in response to perceived aggression from Russia, U.S. Defense Secretary Ashton Carter said Tuesday. Carter's remarks come as tensions have risen between NATO and Russia over the annexation of Crimea as well as Moscow's nuclear ambitions and military buildup along the eastern borders of Europe.

"Moscow's nuclear saber rattling raises troubling questions about Russia's leaders' commitment to strategic stability, their respect for norms against the use of nuclear weapons and whether they respect the profound caution that nuclear-age leaders showed with regard to brandishing nuclear weapons," Carter said.

Russian President Vladimir Putin and other military authorities have repeatedly made veiled statements concerning the country’s willingness to use nuclear weapons, while building up Moscow’s arsenal. “Measures for countering the aggressor could include those that concern Russia’s strategic nuclear capability — that is, reciprocal measures so that, God forbid, no one gets a crazy idea in their head,” Dmitry Rogozin, the deputy prime minister in charge the defense industry, said in October 2015 after the U.S. revealed it was developing new weapons systems, Reuters reported.

Vladimir Putin Russian President Vladimir Putin attends a live broadcast nationwide call-in in Moscow, April 14, 2016. Photo: REUTERS/Michael Klimentyev/Sputnik/Kremlin

Moscow has continued to send military personnel and equipment off the coast of Nordic countries like Finland while completing a military base in the nearby Arctic Circle and building up a military presence near the eastern Baltic states. The Russian military frequently conducts air drills off the coast of Northern European nations, and Carter said these maneuvers are an attempt to intimidate those countries.

Tensions have remained high between the U.S. and Russia since Moscow annexed the Crimean peninsula in 2014 in an act U.S. authorities said violated international law. As a member of NATO, the U.S. has been active in advocating to protect the eastern borders of such countries, including potentially sending a third U.S. army combat brigade as well as 4,000 additional NATO troops to the Baltic states in the coming year.