NATO said Wednesday it will deploy more air, sea and land forces in eastern Europe in response to the worsening crisis in Ukraine.
"Today we have agreed a package of military measures," NATO head Anders Fogh Rasmussen said after a meeting of all 28 members of the transatlantic alliance.
"We will have more planes in the air, more ships on the water... and more readiness on the land," he said, adding that NATO defence plans will be "reviewed and reinforced."
Rasmussen refused to detail what new forces would be deployed and where but said they would result in increased air sorties over the Baltic Sea, while additional ships would go there and to the eastern Mediterranean.
The decision will be implemented "immediately" and there will be more to come, he added.
As the Ukraine crisis has unfolded, NATO has already taken a number of steps, with the United States sending fighter aircraft to the Baltic states and to Poland to bolster confidence in member states once ruled by Moscow.
Rasmussen stressed that NATO would stand by any ally against any threat, while the measures announced were entirely in keeping with international law and the alliance's commitment to deterrence.
He repeated calls for Russia "to be part of the solution" in Ukraine, to stop the destabilisation of its Soviet era satellite, pull back its troops from the border and to make clear it "does not support" extremists.
Asked what bearing the decision could have on EU-US talks with Russia and Ukraine in Geneva on Thursday, Rasmussen said: "We have taken military steps which we think are necessary to enhance deterrence."
At the same time, "we agree that a political solution is the only way forward," he said.