NATO chief Jens Stoltenberg on Wednesday called for avoiding a new arms race, as the US-led alliance pushes ahead with its biggest military revamp since the Cold War to counter a more assertive Russia.

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

Russia however says that it is NATO which is acting aggressively, encroaching on its borders while, worse still, the United States builds a European missile defence shield which undercuts Moscow's nuclear deterrent.

Stoltenberg insisted that while NATO was justifiably boosting its readiness and resources, "it is in everyone's interest to avoid a new arms race."

If NATO was building up its forces, it was also strongly committed to keeping channels for dialogue open with Russia.

"NATO does not seek confrontation," he told a press briefing ahead of a two-day NATO foreign ministers meeting at its headquarters in Brussels.

US Ambassador to NATO Douglas Lute said separately 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" after the Cold War, Lute said.

At the same time, while "we might not have a partner open to dialogue we have to show NATO is always open to dialogue," he added.

NATO suspended all practical cooperation with Russia in 2014 but left what is known as the NATO-Russia Council in place.

Last month, NATO held its first NRC since June 2014, providing what Stoltenberg said was a "frank" but also "useful" exchange with the Russian ambassador to the alliance.

NATO diplomatic sources said some of its 28 member states favour holding another NRC before a July leaders summit in Warsaw, if only to prove the alliance's good faith.

Others are reluctant, believing little was achieved in April and seeing no reason to cut Moscow any slack, the sources said.

The Warsaw summit will sign-off on NATO's increased commitment on its eastern flank, with host Poland in particular pushing a hard line on relations with Russia.

The alliance must also face up to growing security challenges to its South, highlighted by the bloody conflict in Syria right next to key NATO power Turkey.

The situations in North Africa, especially Libya, and in Afghanistan where NATO has wound down its longest ever military campaign are also on the meeting's agenda, along with discussions on how to increase cooperation with the European Union to tackle some of these problems.

Another focal point will be the signing Thursday of an accession accord with Montenegro -- another bone of contention with Russia over the future of the Balkans, home to historic Slav allies and strategic interests.