NATO leaders will this week formally endorse its biggest military revamp since the end of the Cold War at a landmark summit in Warsaw, in the shadow of a resurgent Russia and Britain's shock EU exit vote.

The centrepiece of the summit in the former heart of the Soviet-led Warsaw pact is a "Readiness Action Plan" to counter a more aggressive Russia after Moscow's shock 2014 annexation of Crimea and intervention in Ukraine.

But also important is an EU-NATO cooperation accord, laying out how the alliance -- which includes 22 of the 28 EU member states -- can work with the EU in face of failing states across the Middle East, North Africa and beyond.

Security is heavy, especially around the National Stadium venue, as Poland prepares to welcome world leaders, including US President Barack Obama for his last NATO summit.

Jens Stoltenberg, the Norwegian secretary general of NATO, said this week that the summit "comes at a defining time for our security with threats and challenges from many directions."

But while Russia is still the main theme just as at the last NATO summit in Wales in 2014, it is the vote for a so-called "Brexit" that will be on everyone's lips.

- Brexit 'bad news' -

NATO insists Brexit makes no difference but it clearly adds to uncertainty amid fears Russia may exploit any opening as Britain negotiates its way out of the EU in difficult and complex talks.

The Ukraine crisis proved a rude wake-up call for NATO and the summit is meant to convey a clear message to Moscow that it will not be caught napping again.

The Readiness Action Plan, agreed at the 2014 Wales summit, included a pledge to spend two percent of annual economic output on defence, ending years of cuts.

To reassure nervous east European allies once ruled from Moscow, NATO also tripled the size of its fast Response Force to some 40,000 troops and created a 5,000-strong "Spearhead" unit to deploy within days to any new crisis point.

Poland, the summit host, and the three Baltic states have however pushed hard for more, to get a NATO tripwire presence on the ground to deter any Russian adventurism.

Leaders will accordingly approve sending one multinational battalion each to Poland, Estonia, Latvia and Lithuania, up to 4,000 troops in all.

- Russia, missile defence -

Russia bitterly opposes NATO's expansion into its Soviet-era satellites which it sees as a threat to its own security requiring an appropriate response.

"We don't intend to give in to this militaristic frenzy but it seems that is what they are pushing us to, to provoke a costly and pointless arms race," Russian President Vladimir Putin said last month.

Moscow reserves it direst warnings however for a Ballistic Missile Defence system the United States is building and which the summit is due to declare has reached an initial operating level.

Washington says the shield is designed to counter missile threats from Iran or the Middle East but Russia says that once the system becomes fully operational in 2018, it will undercut its strategic nuclear deterrent.

The NATO upgrade has largely been driven by the Ukraine crisis but leaders are increasingly worried by threats to the south in the shape of massive flows of illegal migrants across the Mediterranean and terror groups such as Islamic State gaining ground in Libya.

NATO leaders are expected to endorse plans to "project stability" by providing advice and assistance in the region to head off future problems.

NATO diplomatic sources say that while the alliance has a role to play, it does not consider itself to be the "first responder" which should be the EU's role.