Russia's ambassador to NATO, Alexander Grushko, accused the Western military alliance of having a "confrontational agenda" and warned that Russia would take countermeasures, in an interview published in the newspaper Kommersant on Thursday.
His comments came as the alliance prepared to kick off a summit in Warsaw that is expected to clear the way for the deployment of more troops to member states near Russia.
Moscow is "offered a confrontational agenda that we are not interested in. NATO must understand that, from a military point of view, all these measures will have only a counterproductive effect. It is clear to every cognizant person that there will have to be a military response from our side," Grushko said.
Russia has long regarded NATO's eastward expansion as a direct threat to its national security.
"The steps that the alliance is taking toward the East are only worsening the situation," Grushko said in the interview. Russia is "especially concerned about the presence of extra-regional powers, primarily the US, in the Black Sea."
NATO, however, has argued that its actions have been proportionate and solely in response to Russian agressiveness.
Russia annexed the Black Sea peninsula of Crimea from Ukraine two years ago in response to that country ousting its pro-Russian president amid a protest movement calling for closer ties with the West.
The annexation and a subsequent pro-Russian separatist rebellion in Ukraine's two easternmost regions have sunk Russia's ties with the West to a low unprecedented since the Cold War.
"The world is a more dangerous place than just a few years ago and NATO is responding with speed and with determination," Stoltenberg said after meeting Polish President Andrzej Duda in Warsaw Thursday. "NATO is ready to protect and defend every part of our territory."
"The alliance never was the aggressor," Duda added, noting that it was important for NATO to show unity and solidarity.
Also on Thursday, US Secretary of State John Kerry visited Ukraine, where he announced that the United States would provide almost 23 million dollars in additional humanitarian assistance to people affected by the conflict in eastern Ukraine.
Kerry will join US President Barack Obama at the two-day NATO summit, which starts Friday. Obama spoke with Russian President Vladimir Putin by phone beforehand.
On Wednesday, Kerry had also visited Georgia, where he signed a defence collaboration deal. He said the US has contributed about 4.3 billion dollars to Georgia and is "working very hard on the strategic onus of the partnership, defence," according to a transcript.
Georgia lost a brief war with Russia in 2008 over two breakaway regions that Russia recognized as independent states, but which Western powers believe to be under Moscow's de facto control.
Both Ukraine and Georgia aspire to join NATO, saying they need to protect their territorial integrity from their former Soviet ruler, Russia.