Nato must bolster its military presence in eastern Europe so it could respond quickly to any threat from Russia, David Cameron has warned.
The Prime Minister has written to his Nato counterparts urging a rethink on relations with Moscow following its "illegal" actions in Ukraine, including the annexation of Crimea earlier this year.
He wrote: "We must review our long-term relationship with Russia. While Nato has only ever sought to be a partner to Russia, not a threat, it is clear that Russia views Nato as an adversary.
"We must accept that the co-operation of recent years is not currently possible because of Russia's own illegal actions in Nato's neighbourhood and revisit the principles that guide our relationship with Russia."
Measures should include sustaining a "robust" defensive presence in eastern Europe, adopting a new schedule of military exercises, positioning equipment and supplies in key locations and boosting Nato's Response Force.
The letter comes days after a report from the House of Commons Defence Committee warned that transatlantic defence forces were not prepared for any threat from Russia.
A Russian tank rolls outside a former Ukrainian military base in Crimea Mr Cameron wants to use next month's Nato summit in south Wales to agree "long-term measures to strengthen our ability to respond quickly to any threat, to reassure those allies who fear for their own country's security and to deter any Russian aggression".
He believes the summit comes at a "pivotal" time in the organisation's history.
"In 2014, the world is more unpredictable than ever," the Prime Minister said. "To the East, Russia has ripped up the rulebook with its illegal annexation of Crimea and aggressive destabilisation of Ukraine. To the South, an arc of instability spreads from North Africa and the Sahel, to Syria, Iraq and the wider Middle East.
"So we must use the summit to agree how Nato should adapt to respond to and deter such threats; and to ensure the continued collective defence of all its members."
Meanwhile, Russia's Foreign Ministry has accused the European Union of withdrawing a ban on supplying Ukraine with military equipment "on the quiet".
Its statement also urged Europe not to be "goaded" by Washington over events in east Ukraine.
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