NATO secretary general Jens Stoltenberg has said he has "no interest" in starting a new Cold War.
But speaking in Brussels on Tuesday, the official said he believes that a "newly assertive" Russia had "destabiliszed" European security.
He warned that NATO would not stand by in the face of future "Russian aggression,", adding that any threat against one NATO member "will be met by force from across the whole alliance, including North America."
His comments in the European Parliament come after NATO defencse ministers last week approved a new multinational force to beef up defencses of frontline alliance members most at risk from Russia.
Stoltenberg said the new unit approved by the United States and NATO's 27 other members will be multinational and rotate in and out of Eastern European member states rather than being based there. Military planners will decide on its composition this spring.
The new force "will be multinational to make clear that an attack against one ally is any attack against all allies and that the alliance as a whole will respond," Stoltenberg told a meeting of the parliament's influential foreign affairs committee.
One proposal thought to be under consideration calls for the creation of a brigade-sized unit: roughly 3,000 troops.
In a wide-ranging speech, Stoltenberg told Mmembers of the European Parliament that Europe and the rest of the world was experiencing threats to its security unseen since the end of the Cold War, but stressed that he had no interest in seeing the start of a new one.
"I do not want a new Cold War, and that is why we will strive for a more constructive dialogue and relationship with Russia," he told the the packed meeting. "After all," he added, "it is better to prevent a crisis than manage one."
"I should make clear that we never suspended political dialogue with Russia and at times of heightened tension, such as we are now seeing, dialogue is even more important.
"Dialogue should not be seen as a weakness, but a sign of self confidence."
Turning to Europe’s defencse capabilities, the Norwegian-born official welcomed increased US defense budget spending, also pointing out that America accounts for a sizeable portion of NATO’s overall budget.
"This is a clear demonstration of America's continued commitment to Europe's security," he argued.
"But European allies are also stepping up to the plate and, in 2015, no less than 16 European allies increased defense spending in real terms. These are promising first steps although the picture is still mixed and there is a long way still to go."
Stoltenberg said he supports efforts to boost Europe's defense industry, saying that another positive sign was the EU "defence action plan."
He said that while there was good cooperation between the EU and NATO — something he said he had prioritiszed at the outset of his mandate — "our impact could be much greater."
The upcoming NATO summit in July was, he argued, an ideal opportunity to reinforce the relationship so that both NATO and the EU could "bring real added value."
"2016 is going to be a big year for cooperation between NATO and the EU," he declared.
NATO's top official also welcomed the agreement thrashed out by the United States and Russia earlier this week for a partial truce in Syria and latest diplomatic efforts to end the five-year-old civil war.
Under the terms of the agreement, the Syrian government and Syria's armed opposition are being asked to agree to a "cessation of hostilities," effective from last Saturday.
Stoltenberg said, "It is now up to all sides to respect the terms of the agreement and ensure that it is implemented."
He also confirmed that NATO is sending a naval group "without delay" to help stop people smugglers in the Aegean Sea, following a request from Germany, Greece and Turkey.
NATO is now directing the standing maritime group to move into the Aegean without delay and start maritime surveillance activities, Stoltenberg said.
Martin Banks covered the European Union, NATO and affairs in Belgium for Defense News.
