After two decades of trying to build a partnership with Russia, the Nato alliance now feels compelled to start treating Moscow as an adversary, an official has said.
Alexander Vershbow, the deputy secretary-general of Nato, said: "Clearly the Russians have declared Nato as an adversary, so we have to begin to view Russia no longer as a partner but as more of an adversary than a partner."
He said Russia's annexation of Crimea and its apparent manipulation of unrest in eastern Ukraine have fundamentally changed the Nato-Russia relationship.
"In central Europe, clearly we have two different visions of what European security should be like," said Mr Vershbow, a former US diplomat and Pentagon official.
"We still would defend the sovereignty and freedom of choice of Russia's neighbours, and Russia clearly is trying to re-impose hegemony and limit their sovereignty under the guise of a defence of the Russian world."
In April, Nato suspended all "practical civilian and military cooperation" with Russia, although Russia has maintained its diplomatic mission to Nato, which was established in 1998.
Mr Vershbow said Nato, created 65 years ago as a bulwark against the former Soviet Union, is considering new defensive measures aimed at deterring Russia from any aggression against members along its border, such as the Baltic states that were once part of the Soviet Union.
"We want to be sure that we can come to the aid of these countries if there were any, even indirect, threat very quickly before any facts on the ground can be established," he said.
To do that, Nato members will have to shorten the response time of its forces, he said.
Mr Vershbow said that among possible moves by Nato is deployment of more substantial numbers of allied combat forces to Eastern Europe, either permanently or on a rotational basis.
For the time being, he said, such defensive measures would be taken without violating the political pledge Nato made in 1997 when it established a new relationship with Moscow.
The terms were aimed at offsetting Russian anger at the expansion of Nato to include Poland and other nations on Russia's periphery.
At the time, Nato said it would not station nuclear weapons or substantial numbers of combat troops on the territory of those new members. Moscow pledged to respect the territorial integrity of other states.
Mr Vershbow argued that Russia has violated its part of that agreement with its actions in Ukraine.
Thus, he said. "we would be within our rights now" to set aside the 1997 commitment by permanently stationing substantial numbers of combat troops in Poland or other Nato member nations in Eastern Europe.
He said that question will be considered by leaders of Nato nations over the summer.