NATO foreign ministers were on Thursday finalising 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.

NATO chief Jens Stoltenberg said the two-day meeting would address "all the important issues" to prepare for a "landmark" summit in Poland in July.

There, NATO leaders will formally endorse the revamp which puts more troops into eastern 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.

"We will discuss how NATO can do more to project stability... and at the same time address how NATO can continue to adapt to a more assertive Russia to find the right balance between defence and dialogue," Stoltenberg told reporters.

US Secretary of State John Kerry, attending the Brussels talks, said NATO was building a "robust" defensive posture on its eastern flank and urged member states to meet pledges to increase defence spending.

The alliance needs to "continue to strengthen our deterrence capabilities through a more robust forward presence," he said.

"NATO is open to a political dialogue with Russia but we will refrain from business as usual until the Minsk commitments are fully implemented," Kerry added.

Under the Minsk process, Moscow agreed to a ceasefire in Ukraine and to halt support for separatist pro-Russian rebels that have carved out an enclave in the east of the country.

- Mutual suspicions -

Stoltenberg told a later press conference that the alliance's AWACS monitoring aircraft could be flown over "NATO territory and international airspace" to help the fight against the jihadist Islamic State group.

In March, IS jihadists killed 32 people in Brussels -- home to the headquarters of both NATO and the European Union as well as a host of diplomatic and corporate offices.

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

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

"I think you have to remember where this started," said a senior US official.

"NATO took these measures because Russia chose to invade and occupy Crimea and then move into eastern Ukraine. The concern... was to ensure that this was not the beginning of a broader move that might threaten NATO territory."

- Avoid new arms race -

In another move likely to infuriate Moscow, NATO signed an accession accord with the tiny Balkan state of Montenegro on Thursday.

Prime Minister Milo Djukanovic said NATO membership was a major step forward for his country "and will help bring about stability in the region and beyond."

Among other states of the former Yugoslavia, Croatia and Slovenia have joined NATO to Russia's anger over the future of the Balkans, a key strategic interest and home to historic Slav allies.

Georgia, which fought a brief 2008 war with Russia, is also seeking membership but when asked Thursday if Tbilisi could expect similar progress, Stoltenberg notably stopped short of commenting directly on its accession prospects.

Instead, he stressed NATO would continue to boost cooperation, including military training, with the former Soviet republic.

Stoltenberg had cautioned Wednesday against a new arms race, stressing the alliance upgrade was purely "defensive, proportionate and in line with our international obligations."

NATO wants 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).

The NATO talks conclude on Friday.