NATO took the first step Wednesday to boost its deterrence against Russian aggression in the east of the alliance, as it seeks to juggle what Secretary General Jens Stoltenberg called the "most challenging" security threats in a generation.
Relations between NATO and Russia in the last two years have reached their worst level since the Cold War, following Moscow's actions in Ukraine, where it is accused of supporting pro-Russian separatists and has annexed the Crimean peninsula.
But the alliance must also stay "agile" and be prepared for other threats, Stoltenberg warned. NATO is confronted with challenges including the conflict in Syria, the migration crisis this has triggered and the spread of terrorism.
NATO has previously taken steps to reassure its eastern members, who have felt particularly threatened by Russia. The new plan aims to go further, with a combination of flexible ground forces and rapidly deployable reinforcements to deter any potential aggressor.
"NATO defence ministers agreed on an enhanced forward presence in the eastern part of our alliance," Stoltenberg said following talks in Brussels. "This will be multinational to make clear that an attack against one ally is an attack against all," he added.
The new defence framework will include rotational troop deployments, exercises and infrastructure to pre-position equipment and rapidly deploy reinforcements, Stoltenberg said.
The details - including troop numbers and national contributions - are to be hashed out by military planners over the coming months, ahead of NATO's Warsaw summit in July.
As part of this effort, US President Barack Obama announced his intention last week to increase Washington's funding of European security fourfold in 2017, to 3.4 billion dollars. The United States hopes other allies will follow suit.
The new initiative could see up to six thousand troops deployed on a rotational basis across Poland, Bulgaria, Romania and the Baltic states, NATO sources have said on condition of anonymity. The alliance is already setting up regional hubs in these countries, but they are each staffed with around 50 soldiers.
"We will have as much presence in the East as needed," Stoltenberg said Wednesday, while adding that the aim is to strike a balance between troops on the ground and quickly deployable reinforcements, as well as hoping to keep open a dialogue with Moscow.
The alliance has been at pains to respect an agreement struck with Russia in 1997 to refrain from the "additional permanent stationing of substantial combat forces" in Eastern Europe.
But Russian Foreign Minister Sergei Lavrov accused NATO of inflating fears of a Russian threat to justify its existence and to expand eastwards, in comments to the Moskovsky Komsomolets newspaper, carried by TASS news agency.
NATO should "stay within its borders," he said, while criticizing the alliance for deciding last year to invite Montenegro to join. NATO's actions are creating a "destabilizing factor for global stability," Lavrov warned.