Nato defence ministers have approved a new multinational force to beef up defences of frontline alliance members most at risk from Russia, the alliance's secretary general has said.
Jens Stoltenberg said the new unit will be multinational and will rotate in and out of Eastern European member states rather than being based there.
He said military planners will decide on its composition during the spring.
The new force's multinational composition, Mr Stoltenberg said, will make it clear that for Nato, "an attack on one ally will be an attack on the whole alliance".
Getting firm commitments, or even deciding how many Nato troops should be rotated eastward, may take time, however.
Douglas Lute, US ambassador to Nato, said he expected ministers this week to agree on "a framework" but that actual force levels will probably be hammered out only after consultations with Nato's supreme commander in Europe, US Force General Philip Breedlove.
One Nato official said one proposal being considered calls for the creation of a brigade-sized unit: roughly 3,000 troops.
Reinforcing Nato's presence in Poland and other allies close to Russia "will send a clear signal," Mr Stoltenberg said earlier on Wednesday. "Nato will respond as one to any aggression against any ally."
In the discussions held at Nato headquarters in Brussels, US defence secretary Ash Carter and his counterparts from Canada and the alliance's European members are also expected to discuss what other countries can do to better deter Russia following the Obama administration's February 2 announcement that it wants to quadruple spending on US troops and training in Europe.
A senior Nato official said the US hopes its European Nato partners will commit to new investments for deterrence that would correspond to the 3.4 billion US dollars (£2.3 billion) in extra spending and troops, equipment and training moves the Pentagon is proposing.
The alliance's new blueprint for deterrence relies on rotating some forces through frontline member states and prepositioning supplies there, but also on Nato's capacity to airlift in large numbers of reinforcements quickly in a crisis. It may only be fully ready when alliance heads of state and government meet in July.
"It is now less than six months until our next summit meeting, in Warsaw," Mr Stoltenberg reminded defence ministers on Wednesday. "There remains a great deal to do."
Following a request by Turkey, ministers are also reviewing what the US-led alliance could do to help slow the influx of migrants into Europe by sea, Mr Stoltenberg said.
"We all understand the concern and we all see the human tragedy," Mr Stoltenberg said. He said the discussions could lead to a decision to use Nato air or sea assets to help combat people-smuggling.
The International Organisation for Migration (IOM) on Tuesday said 409 people have died so far this year trying to cross the Mediterranean Sea, and that migrant crossings in the first six weeks of 2016 are running at nearly 10 times the rate of the same period last year. IOM said 76,000 people have reached Europe by sea, nearly 2,000 per day, since January 1.
Germany, the leading destination for the migrants, many of whom are fleeing war or poverty in their homelands in the Middle East or Africa, welcomed the discussions at Nato.
"It is good that the Turkish government has asked Nato to help for the surveillance of the sea. We are aiming at stopping the business of the smugglers," German defence minister Ursula von der Leyen said as she arrived at alliance headquarters.