NATO will deploy four battalions to Poland and the Baltic countries as part of its response to Russia's actions in Ukraine, the chief of the Western military alliance said Monday.
The move is likely to ratchet up tensions with Moscow, which has repeatedly warned that an eastward NATO expansion threatens its national security.
"NATO has taken robust action to protect our nations and to contribute to stability in our neighbourhood. But the challenges we face are enduring," NATO Secretary General Jens Stoltenberg said, one day before defence ministers were due to discuss the deployment.
"Based on the advice of our military planners, we will agree to deploy by rotation four robust multinational battalions in the Baltic states and in Poland," Stoltenberg added. "This will send a clear signal that NATO stands ready to defend any ally."
The military alliance's eastern-most members have said that they feel threatened by Russia, which has been accused of violating international law by annexing the Ukrainian peninsula of Crimea. Its support for separatists in eastern Ukraine has also raised concerns.
Each battalion will feature "a combat-ready force of 800 to 1,000 troops" which will be on rotation for six to nine months, according to the US ambassador to NATO, Douglas Lute.
"The idea here is to establish a forward presence, which confirms that any encroachment, any attack on those four allies will immediately ... make contact with NATO forces and trigger rapid responses from the rest of the alliance," Lute said.
The rotations will be staggered so that there will be a troop presence "365 days a year," he added.
The United States will lead one battalion, while Britain and Germany are expected to head two others. Who will lead the Poland deployment still has to be confirmed, but a diplomat said on condition of anonymity that Canada may be a contender.
Lead nations will "take responsibility to frame the battalion and provide the bulk of the battalion forces," Lute said.
NATO had struck an agreement with Russia in 1997 to refrain from the "permanent stationing of substantial combat forces" in Eastern Europe. Stoltenberg said NATO's actions are "fully in line with our international obligations and commitments."
"We can take sufficient and necessary steps without locking ourselves into some kind of a spiral with Russia," Lute added, arguing that the actions under consideration are not "a threat to anybody."
"You don't invade with a few battalions," he said. "But you can deter and you can affect a potential aggressor's calculus in terms of cost, benefit and risks."
The defence ministers will "fine-tune" the plans so they can be announced by NATO leaders at their next summit in July, Lute said. By then, the lead nations and other contributors, the start of the deployments and their exact locations will be clarified.
Stoltenberg said the deployments will be open-ended, with no end date decided upon yet.
The ministers, who will meet for two days in Brussels starting Tuesday, will also discuss the fight against the Islamic State extremist group, the crisis in Ukraine, the situation in Afghanistan, cooperation with the European Union and defence spending.
NATO estimates that defence expenditures in European member states and Canada will increase in 2016 for a second consecutive year, Stoltenberg said on Monday, calling it "real progress."