Wales: NATO will bolster its eastern defences and buttress support for Kiev at a summit starting on Thursday, prompted by the Ukraine crisis to enact the most radical shift in its strategy towards Russia since the end of the Cold War.
U.S. President Barack Obama and other NATO leaders meeting at the Celtic Manor resort in south Wales will also discuss how to tackle Islamic State which has emerged as a new threat on the alliance`s southern flank.
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, which is not a member.
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.
In an opinion piece for The Times in Britain on Thursday, Obama and British Prime Minister David Cameron wrote: "We meet at a time when the world faces many dangerous and evolving challenges. 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 will discuss the crisis in Ukraine with President Petro Poroshenko on Thursday.
They are expected to approve a package of support, setting up trust funds expected to be worth around 12 million euros ($15.8 million) to improve Ukrainian military capabilities in areas such as logistics, command and control and cyber defence.
As part of a stepped-up programme of war games, 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 NATO 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. So we have to now figure out what kind of relationship we can have with Russia," 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 eastern Ukraine.
Obama stopped off in Estonia on his way to the summit in a symbolic show of support for the former Soviet republic, now a NATO member.
What NATO leaders will agree to do to help Iraq combat Islamic State 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 some individual allies could choose to do so.
Leaders will study whether NATO could play a role in helping coordinate aid being given by individual allies in supplying humanitarian aid or military equipment, a British government source said.
They could also look at cooperating more on intelligence and surveillance about Islamic State`s activities, the source said.
NATO could also revive a previous mission to help train the Iraqi armed forces that it halted in 2011, diplomats say.