NATO leaders agreed on Friday to deploy military forces to the Baltic states and eastern Poland for the first time, and also to increase air and sea patrols to reassure allies who were once part of the Soviet bloc following Russia's seizure of Crimea from Ukraine.
The 28-nation Western defence alliance decided to move four battalions, totalling 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 neighbours."
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 today, where he may face pressure to fulfil Kiev's part of the agreement by accepting more decentralisation and local elections in the rebel-held eastern Donbass region.
German Chancellor Angela Merkel said that while NATO was increasing its defence 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.
Adversary and partner
Coinciding with the NATO summit, the US state department announced it had expelled two Russian diplomats on June 17, in response to an attack by a Russian policeman on a US diplomat in Moscow earlier last month. It was not clear why the United States waited until Friday to disclose the news.
Estonia, Latvia, Lithuania and Poland had requested a permanent NATO presence, fearing Moscow will seek to destabilise their pro-Western governments through cyber attacks, stirring up Russian speakers, hostile broadcasts or territorial incursions.
The three Baltic states, as well as Ukraine, are former Soviet republics that gained independence when the Soviet Union broke up in 1991.
The head of NATO's military committee, Czech general Petr Pavel, said Russia was attempting to restore its status as a world power, an effort that included using its military. "We must accept that Russia can be a competitor, adversary, peer or partner and probably all four at the same time," he said.
The Kremlin said it was absurd for NATO to talk of any threat from Russia and it hoped "common sense" would prevail at the Warsaw summit. Moscow remains open to dialogue with NATO and is ready to cooperate with it, Kremlin spokesman Dmitry Peskov said in a conference call with journalists.
Russia often depicts NATO as an aggressor whose members are moving troops and military hardware further into former Soviet territory, which it regards as its sphere of influence.
Russian President Vladimir Putin discussed diplomatic efforts for a settlement in Ukraine in a phone call with Merkel and French President Francois Hollande just before the summit began. The Kremlin said he asked them to "influence more actively the Ukrainian side" to grant wider autonomy to eastern Ukraine.
Moscow has declared its intention to deploy nuclear-capable missiles in Kaliningrad, a Russian enclave between Poland and Lithuania, in response to NATO's activation of a US-built missile shield on Polish soil.
Outgoing British Prime Minister David Cameron vowed Britain would not turn its back on European security once it leaves the EU following the Brexit vote. Britain is Europe's biggest military spender, followed by France.
Hollande, who has sent French forces on missions against Islamist militants in Mali, the Central African Republic, Iraq and Syria, urged other European allies to increase their defence budgets - veiled criticism of Germany, which spends just over one per cent of GDP on the military, or half the NATO objective.
NATO and the EU signed an agreement on deeper military and security cooperation. The US-led alliance is set to announce today its support for the EU's Mediterranean interdiction operation. NATO is already backing EU efforts to stem a refugee influx from Turkey into Greece, in conjunction with an EU-Turkey deal to curb migration in return for benefits for Ankara.
Stoltenberg also said NATO defence ministers would consider calls from Romania and Bulgaria for a stronger allied air and sea presence in the Black Sea, where Russia has a fleet based in Crimea and is building up its interdiction capacity.