NATO says it is suspending military and civilian co-operation with Russia over its annexation of Ukraine’s Crimea region.
It has also ordered military planners to boost defences and reassure nervous eastern European countries.
The tough response comes as foreign ministers of the 28-nation alliance met for the first time since Russia’s intervention in Crimea triggered the worst East-West crisis since the Cold War.
NATO Secretary-General Anders Fogh Rasmussen said Russia’s actions meant there could be no “business as usual”.
“Russia’s aggression against Ukraine is the gravest threat to European security in a generation and it challenges our vision of a Europe, whole, free and at peace,” said Rasmussen.
Ministers are also questioning Moscow’s claim to have withdrawn troops from near the Ukrainian border
US Secretary of State John Kerry said although the preference was for de-escalation, the NATO alliance took seriously any attempt to change borders by use of force:
“Together we have rejected any notion that there is any legality in Russia’s efforts to annex Crimea and we’ve all challenged the tactics of intimidation, particularly the deployment of unprecedented amounts of military forces around Ukraine’s borders,” he said.
The US and its allies have offered more planes to take part in regular NATO patrols over the Baltic States which were once Soviet republics. Washington is also looking at bolstering a scheduled training exercise with Poland.
But despite the uncompromising stance NATO’s chief stressed the only path to follow was the political and diplomatic one.