NATO leaders were set to buttress support for Kiev and bolster their eastern defences at a summit starting on Thursday, spurred by the Ukraine crisis to enact the most radical shift in allied strategy towards Russia since the end of the Cold War.
U.S. President Barack Obama and his 27 allies meeting at a golf resort in Wales will also discuss how to tackle the Islamic State straddling parts of Iraq and Syria, which has emerged as a new threat on the alliance's southern flank, and how to stabilise Afghanistan when NATO forces leave at year's end.
British Prime Minister David Cameron, the summit's host, said pressure on Moscow would mount if it did not curtail military action in Ukraine which he branded unacceptable.
"What Russia needs to understand is if they continue with this approach in Ukraine, this pressure will be ramped up," he told BBC television, adding that U.S. and EU sanctions were already having an effect on the Russian economy.
Ukrainian President Petro Poroshenko, whose forces have suffered a string of setbacks at the hands of Russian-backed separatists in the south and east of the country in the last week, was to meet Obama and the leaders of Britain, France, Germany and Italy just before the NATO summit starts.
The Ukrainian leader is looking for arms, training and intelligence support for his armed forces as well as political support against Russian President Vladimir Putin.
However, his talk of reviving Ukraine's bid to join the U.S.-led military alliance could reopen a rift within NATO.
Obama said in Estonia on Wednesday the door to membership would remain open to states that meet NATO standards and "can make meaningful contributions to allied security", but France and Germany remain opposed to admitting Kiev.
A French official said NATO should contribute to easing tensions, not exacerbating them.
Russian Foreign Minister Sergei Lavrov underlined Moscow's opposition to Ukraine joining NATO, saying attempts to end the country's non-aligned status could "derail all efforts aimed at initiating a dialogue with the aim of ensuring national security".
After a week of defiant statements from Putin, Lavrov said Russia was ready for practical steps to de-escalate the crisis in Ukraine and urged Kiev and the rebels to heed ceasefire proposals put forward by Moscow on Wednesday.
As more than a decade of NATO-led combat operations in Afghanistan draws to a close at year's end, the 28-nation, U.S.-led military alliance is refocusing in part on its core task of defending its territory.
NATO leaders will set up a "spearhead" rapid reaction force, potentially including several thousand troops, that could be sent to a hotspot in as little as two days, officials say.
Eastern European NATO members, including Poland, have appealed to NATO to permanently station thousands of troops on its territory to deter any possible Russian attack.
But NATO members have spurned that idea, partly because of the expense and partly because they do not want to break a 1997 agreement with Russia under which NATO committed not to permanently station significant combat forces in the east.
Instead, NATO leaders will agree to pre-position equipment and supplies, such as fuel and ammunition, in eastern European countries with bases ready to receive the NATO rapid reaction force if needed.
NATO has said it has no plans to intervene militarily in Ukraine. Instead it has concentrated on beefing up the defences of eastern European countries that have joined the alliance in the last 15 years. Baltic countries fear that Russia could use the same rationale it applied in Crimea - defending Russian speakers - to meddle with them.
"RIPPED UP THE RULEBOOK"
In an article in The Times newspaper on Thursday, Obama and British Prime Minister David Cameron wrote: "To the east, Russia has ripped up the rulebook with its illegal, self-declared annexation of Crimea and its troops on Ukrainian soil threatening and undermining a sovereign nation state.
"To the South, there is an arc of instability that spreads from North Africa and the Sahel, to the Middle East."
People who wanted to take an isolationist approach to such threats "misunderstand the nature of security in the 21st century", they wrote. "Developments in other parts of the world, particularly in Iraq and Syria, threaten our security at home."
NATO leaders are expected to approve a package of support for Kiev, setting up trust funds worth around 12 million euros ($15.8 million) to improve Ukrainian military capabilities in areas such as logistics, command and control and cyber defence.
A dozen countries will take part in an exercise in Lviv, Ukraine, later this month, co-hosted by Ukraine and the U.S. Army.
NATO officials say the alliance itself will not send the weapons that Ukraine is looking for. But individual allies could do so if they wished.
NATO leaders will also discuss the alliance's relationship with Russia, which officials say has been fundamentally changed by the Ukraine crisis.
After the end of the Cold War, NATO and Russia sought cooperation in some security fields but NATO has concluded that this effort has failed, and for now at least, Russia is not a partner, a senior official said.
"Russia has basically violated very fundamental agreements on the basis of which we have constructed peace and security in Europe for the last two decades," the official, speaking on condition of anonymity, said.
NATO has already suspended cooperation with Moscow following its annexation of Ukraine's Crimea region.
France, which has faced fierce pressure from Washington and other NATO allies to halt the sale of two helicopter carriers to Russia, said on Wednesday it would not deliver the first of the warships for now because of Moscow's actions in Ukraine.
What NATO leaders will agree to do to help Iraq combat Islamic State militants is less clear.
The alliance as a whole is highly unlikely to follow the U.S. lead in staging military strikes on Islamic State, NATO diplomats say, although individual allies such as France and Britain may do so. NATO could revive a mission to help train the Iraqi armed forces that it halted in 2011, diplomats say.
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