NATO defence ministers on Tuesday approved sending battalions to the three Baltic states and Poland just weeks before a landmark summit in Warsaw endorses a major build-up to counter a more assertive Russia.

Russia's 2014 intervention in Ukraine and its annexation of Crimea stung NATO out of its post-Cold War complacency and into a major revamp to boost its readiness and resources to meet a host of new security challenges.

NATO chief Jens Stoltenberg said ministers agreed to deploy four "robust" multinational battalions to Latvia, Lithuania, Estonia and Poland -- all once ruled from Moscow and deeply suspicious of Russian intentions.

Additionally, "we will take tailored measures to enhance our defence and deterrence in the Black Sea region," Stoltenberg said, citing a Romanian offer to host a similar unit.

"This sends a clear message. If any of our allies is attacked, the whole alliance will respond as one," he said.

Stoltenberg stressed NATO did not seek confrontation with Russia and wanted a constructive dialogue but it would defend the 28 allies against any threat.

- Russian warnings -

Russia bitterly opposes NATO's expansion into its Soviet-era satellites and last month said it would create three new divisions in its southwest region to meet what is described as a dangerous military build-up along its borders.

NATO opened a missile defence base in Romania last month, sparking furious Russian warnings that this endangered its nuclear deterrent and it would have to consider retaliatory measures.

An increased NATO presence in Romania is additionally sensitive given that Russia's key Black Sea fleet has its historic base in Crimea.

Diplomatic sources say the four battalions in the Baltic states and Poland are likely to number 2,500-3,000 troops in total, with the small force designed to act as a tripwire once deployed from next year.

Officials said the United States, Britain and Germany had agreed to be lead nations for the battalions, with Canada expected to be the fourth.

- Cyber defence upgrade -

The Ukraine crisis also highlighted the new danger posed by "hybrid warfare," a mixture of conventional weaponry and information technology to weaken and destabilise an opponent without a formal declaration of hostilities.

Many in NATO were shocked by the speed and effectiveness of Russia's intervention in Ukraine and its annexation of Crimea, and Stoltenberg said ministers had agreed to boost efforts to face the threat.

"We agreed that we will recognise cyberspace as an operational domain, just like land, sea and land," Stoltenberg told reporters at NATO HQ in Brussels.

"Most crises and conflicts today have a cyber dimension so treating cyber as an operational domain will enable us to better protect our missions and operations," he said.

NATO recognised two years ago that a cyber attack against a member state could be considered the equivalent of a military attack, triggering its 'one for all, all for one' Article 5 collective defence commitment.

- More defence spending -

Stoltenberg said NATO had now completed the "Readiness Action Programme" adopted at its 2014 leaders' summit in Wales, boosting resources and sharply improving response times to ensure it would not get caught napping again.

NATO leaders meet in Warsaw July 8-9 to sign off on the upgrade, which was topped off with a crucial commitment by member states to increase defence spending to two percent of annual economic output.

Stoltenberg said the alliance had finally reversed years of defence cuts in 2015, with spending up 0.6 percent and expected to increase another 1.5 percent in 2015.

"This is progress but I call on the allies to keep up the momentum and to do more because we need to match our defence spending with the challenges we face," he said.

The Warsaw summit will be hugely symbolic since the Polish capital gave its name to the Soviet-era Warsaw Pact, NATO's military adversary for nearly 50 years until the fall of Communism.