Prime Minister David Cameron has said Nato must rethink its long-term relationship with Russia in the wake of its "illegal" actions in Ukraine.
He also called for Nato's members to take measures to ensure it can respond quickly to any threat from Moscow.
In a letter to his Nato counterparts, Mr Cameron said the measures should include sustaining a "robust" defensive presence in eastern Europe, keeping equipment and supplies in key locations and beefing up Nato's Response Force of swiftly deployable land, air, maritime and special operations troops.
The letter comes just days after a government report warned the trans-Atlantic military alliance was not adequately prepared for a potential threat from Russia.
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