The 28-nation Western defense alliance decided to move four battalions totaling 3,000 to 4,000 troops into northeastern Europe on a rotating basis to display its readiness to defend eastern members against any Russian aggression.
However, they also underlined their willingness to pursue a dialogue with Moscow and revive confidence-building measures that Russia has spurned since its 2014 annexation of Crimea and support for Russian-speaking rebels in eastern Ukraine.
"These battalions will be robust and they will be multinational. They make clear that an attack on one ally will be considered an attack on the whole alliance," NATO Secretary-General Jens Stoltenberg told a news conference after the summit's first working session in Warsaw, the Polish capital.
President Barack Obama said the United States would deploy about 1,000 soldiers in Poland under the plan "to enhance our forward presence in central and eastern Europe". Germany will lead the battalion in Lithuania, Britain in Estonia and Canada in Latvia. Other nations such as France will supply troops.
White House deputy national security adviser Ben Rhodes told reporters that what he called continued aggression by Russia would provoke a response by NATO and a greater alliance presence in Eastern Europe.
Obama said earlier that Britain's referendum vote to leave the European Union, an outcome he sought to avoid, should not weaken the Western alliance but raised "significant questions" about the future of European integration. America's "special relationship" with the UK would survive, the president said.
Obama discussed the procedure for Britain's withdrawal with the heads of the main EU institutions, Donald Tusk and Jean-Claude Juncker, and was assured there would be an orderly transition to as close an economic relationship as possible and no punishment of Britain, Rhodes told reporters.
Host nation Poland set a tone of mistrust of Russian intentions. Foreign Minister Witold Waszczykowski told a pre-summit forum: "We have to reject any type of wishful thinking with regard to pragmatic cooperation with Russia as long as it keeps on invading its neighbors."
Obama was more diplomatic, calling for dialogue with Russia, but he too urged allies to keep sanctions on Moscow in place until it fully complies with a ceasefire agreement in Ukraine.
Ukraine is not a member of NATO but President Petro Poroshenko will meet allied leaders on Saturday, where he may face pressure to fulfill Kiev's part of the agreement by accepting more decentralization and local elections in the rebel-held eastern Donbass region.
German Chancellor Angela Merkel said that while NATO was increasing its defense capabilities, it was always keen for dialogue with Moscow. A planned meeting of the long frozen NATO-Russia Council next week would address ways to avoid dangerous situations in Baltic air space, she said.
Russian warplanes have been buzzing Western civilian and military aircraft and switching off their identification signals as part of an apparent campaign of intimidation in response to Western economic sanctions over its action in Ukraine.
"Just as there are understandings between the United States and Russia in Syria, it's in both sides' interests that NATO and Russia also coordinate their activities," Merkel said.