Nato tries to reassure eastern members with new 'spearhead' force
A new "spearhead" force which can deployed rapidly is among several measures agreed at Nato's summit to reassure eastern members worried by Russia's actions in Ukraine.
The force, numbering several thousand, could be deployed within days, Nato chief Anders Fogh Rasmussen said.
Supplies and equipment will also be pre-positioned in eastern Nato states.
Nato accuses Russia of sending arms and troops to back the rebels in eastern Ukraine. Moscow denies this.
Nato members, such as the Baltic states and Poland, have been particularly concerned about their security since the Ukraine crisis erupted earlier this year.
Previously part of the Soviet Union, the Baltic states fear a resurgent Russia could use the pretext of protecting Russian speakers to intervene.
The decision on a new force - rumoured to number up to 5,000 - sent "a clear message", Mr Rasmussen said.
"Nato protects all allies at all times."
The force would deploy ground troops with air, naval and special forces support. In a further demonstration of resolve, the alliance, founded in 1949 to counter the threat from the Soviet Union, adopted a Readiness Action Plan.
Analysis: Jonathan Marcus, BBC defence and diplomatic correspondent
Nato's new Readiness Action Plan is the practical military response to the growing threats from around Nato's borders; not just from a resurgent Russia but also from the growing tide of instability in the Middle East.
It is all about being able to get more capable forces to areas of threat faster than ever before.
Existing rapid reaction forces will be re-vamped. A multi-national "spearhead" force with air, sea and land elements is to be established, capable of reinforcing a Nato ally within 48 hours.
This will require some headquarters, logistics and other elements to be established in eastern Nato member states and some supplies will need to be pre-positioned.
These reinforcement plans will also be extensively rehearsed through a stepped up pattern of exercises.
Intelligence gathering and analysis will be stepped up to give Nato planners a greater awareness of what is going on around the alliance's borders.
The UK has offered to contribute 1,000 personnel to the "spearhead" force.
The alliance, whose 28 leaders met for two days in Wales, also agreed to make cyber defence part of its core mission.
Cyber attacks "could harm our modern societies as much as a conventional attack", Mr Rasmussen said.
In addition, Nato will increase military exercises in eastern Europe. Rotation of air, sea and land forces through the region will continue.
Relations between Nato and Russia have declined markedly since the turn of the century.
In 2000, Russian President Vladimir Putin even raised the possibility of joining Nato, recalls the BBC's Steve Rosenberg in Moscow, but since then the Kremlin feels its views have been ignored.