NATO's build-up in eastern Europe could include up to 3,500 troops, Britain said on Friday, stressing that the planned deployments would not be aggressive toward Russia.
Russia's seizure of Crimea in 2014 has prompted the Western military alliance to consider deterrent forces in the Baltics and Poland which British Foreign Secretary Philip Hammond said would be a "trip wire" to alert NATO of any potential threat.
NATO defense ministers are expected to decide on the troop levels next month, while making clear no large forces will be stationed permanently, to avoid provoking the Kremlin.
"It looks like there could be four, maybe five battalions ... the point of these formations is to act as a trip wire," Hammond told reporters.
"It isn't intended to be aggressive," he said following a meeting of NATO foreign ministers in Brussels.
Hammond said that could amount to as many as 3,500 troops along NATO's border with Russia, with Britain, Germany and the United States taking the bulk of command duties.
In total, the deterrent will be made up of small eastern outposts, forces on rotation, regular war games and warehoused equipment ready for a rapid response force which would include air, maritime and special operations units.
NATO diplomats say the United States is likely to command two battalions, with Britain and Germany taking another each. That leaves a fifth NATO nation to come forward to lead the remaining battalion, with Denmark, Spain, Italy or the Netherlands seen as possible candidates, diplomats say.
The force build-up follows a speech by U.S. President Barack Obama in Estonia in 2014 in which he said NATO would help ensure the independence of the three Baltic states, which for decades were part of the Soviet Union.
NATO foreign ministers said they had agreed to propose to Moscow another meeting of the NATO-Russia Council, which met in April for the first time in nearly two years, to set out what the alliance says is a proportionate response to Russia's annexation of Crimea.
NATO suspended all practical cooperation with Russia in April 2014 in protest over Crimea. NATO said high-level political contacts with Russia could continue but NATO and Russian ambassadors have met only three times since.
"We are doing things that could be misinterpreted," Hammond said. "We judged that creating an opportunity through the NATO-Russia Council is the best way of avoiding Russia being able to say: 'we haven't been informed, we didn't know the details.'"