NATO head Jens Stoltenberg said Thursday the alliance must counter a Russian military build-up in the Baltic, the Black Sea and the eastern Mediterranean which could give Moscow control of key areas in a crisis.

He said the 28-nation, US-led alliance must also consider doing more to reassure eastern member states once ruled from Moscow who have been badly unnerved by Russia's intervention in Ukraine.

As ties with the west have deteriorated, Russia has boosted its military presence in its Kaliningrad enclave, which sits west of and on the blind-side of the Baltic states.

Moscow has meanwhile deployed troops, aircraft and navy ships to Syria to bolster long-time ally President Bashar al-Assad.

Stoltenberg warned that Russia is acquiring the ability and presence to exercise control over strategic points and NATO must ensure it can carry out its own missions in such a changed environment.

"This is a military build-up which provides the Russians with what many experts call Anti-Access/Area Denial (A2/AD) capabilities," he told reporters at the Portuguese naval base of Troia south of Lisbon.

"We have to be sure we are able to overcome these capabilities so we can reinforce and deploy forces if needed," he said, after watching troops take part in the Trident Juncture exercise, NATO's biggest in more than a decade.

"The question on our agenda now is how to overcome, how to deal with the increased A2/AD capabilities of Russia in the Baltic, the Black Sea and now in the Mediterranean."

- Stepped-up presence -

Stoltenberg made his remarks when he was asked about what more NATO should do to reassure eastern member states who fear for their future in the face of a more assertive Russia.

They want NATO to do more and have suggested the alliance could even set up permanent bases on their soil.

NATO has previously ruled out that possibility for fear of breaching treaties agreed with Russia banning such a presence.

But Stoltenberg said Thursday there was no real distinction to be made between permanent and the sort of temporary, rotating NATO deployments of troops, ships and planes which have all been stepped up since the Ukraine crisis broke.

NATO has also set up forward command units in Estonia, Latvia, Lithuania, Hungary, Bulgaria and Romania, and pre-positioned equipment so that its new high speed rapid reaction force can hit the ground running in any crisis.

"We have already increased our presence and we are looking into the question of whether we should increase it even more," Stoltenberg said, adding that the issue would be on the agenda for the next NATO leaders summit in Warsaw in July 2016.

Stung by Russia's intervention in Ukraine, NATO leaders agreed last year to reverse years of defence spending cuts and to upgrade its rapid response force, more than doubling its size to around 40,000 troops.

They also approved what is known as the Very High-Readiness Joint Task Force (VJTF), a smaller "spearhead" unit which can put boots on the ground within 48 hours.

The Trident Juncture exercise is NATO's biggest since 2002, putting some 36,000 troops through their paces over five weeks in Italy, Spain and Portugal.