NATO foreign ministers meet Thursday to finalise the alliance's biggest military build-up since the end of the Cold War to counter what they see as a more aggressive and unpredictable Russia.

At a Warsaw summit in July, NATO leaders will sign-off on the revamp which puts more troops into east European member states as part of a "deter and dialogue" strategy, meant to reassure allies they will not be left in the lurch in any repeat of the Ukraine crisis.

Ministers will also discuss the growing challenges on NATO's southern flank, from conflict in Syria and Iraq to instability across North Africa amid fears terrorist groups such as Islamic State (IS) can exploit the turmoil.

In November, IS attacks in Paris left 130 dead and fellow jihadis followed that up with more killings in March in Brussels -- home to NATO HQ, the European Union and a host of diplomatic and corporate offices.

The EU is also grappling with the worst migrant crisis since the end of World War II and the bloc is anxious to increase cooperation with NATO to tackle the problem, notably trying to bolster the UN-backed government in Libya where IS has recently gained ground.

EU foreign affairs head Federica Mogherini will join the NATO ministers on Friday to discuss the possibilities.

Of the 28 NATO member states, 22 also belong to the EU where Mogherini is overseeing a review of the bloc's global security strategy.

- 'Sober discussion' on Russia -

US Ambassador to NATO Douglas Lute said Wednesday foreign ministers would have a "very sober discussion on dealing with Russia... which essentially has thrown out the rulebook."

"This is not the predictable partner we thought we had," Lute said, adding: "We might not have a partner open to dialogue but we have to show NATO is always open to dialogue."

A focal point is the signing Thursday of an accession accord with Montenegro -- yet another bone of contention, this time over the future of the Balkans, home to historic Slav allies and a key strategic interest for Russia.

Russia's intervention in Ukraine and its 2014 annexation of Crimea stung NATO into action after years of complacency and defence cuts following the fall of the Soviet Union.

Moscow however says NATO is encroaching on its borders, while Washington builds a European missile defence shield which undercuts Russia's nuclear deterrent.

Mutual suspicion runs deep -- former NATO deputy supreme commander Richard Shirreff warned Wednesday the West could find itself at war with Russia next year unless it boosted its defences.

His book, "2017 War With Russia," is based on what the publishers said were NATO "war gaming scenarios" -- in this case, Russia opening a land corridor to Crimea and seizing the Baltic states.

James Stavridis, NATO supreme allied commander for Europe in 2009-13, said Shirreff got it right about Russian action in Crimea and eastern Ukraine and was likely right again now.

- Avoid new arms race -

NATO chief Jens Stoltenberg warned Wednesday against the dangers of a new arms race, stressing the alliance's response was purely "defensive, proportionate and in line with our international obligations."

NATO wanted dialogue with Russia to ease tensions and avoid potentially dangerous incidents getting out of control, he said.

NATO suspended all practical cooperation with Russia over Ukraine but left a channel of communication open through what is known as the NATO-Russia Council (NRC).

Stoltenberg convened the first NRC since June 2014 last month, which he said produced a "frank" but also "useful" exchange.

NATO diplomatic sources said some member states wanted another NRC before the Warsaw summit, as at least a gesture of good faith, but others are reluctant, seeing no reason to cut Russia any slack.

"I think there will be a meeting... a number of allies want it quite badly and the rest of us think it is not worth fighting about," one source said, downplaying the NRC's importance.