UK Foreign Secretary Philip Hammond said Nato needs to show a political will to fight back if Russia was to attack any of the alliance's members.
Mr Hammond said Nato leaders needed to make clear there was a "red line" around the 28 member states which could not be crossed.
His comments come ahead of a Nato summit in Wales this week.
Meanwhile, several former Eastern bloc member states - such as Estonia, Latvia and Lithuania - warned they could be the next target of Russian aggression following Moscow's incursions into eastern Ukraine.
Estonian President said there should be a permanent Nato base in his country's territory.
Under the Nato treaty, the whole alliance would be obliged to defend any member state if they came under an attack.
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