WARSAW (AFP) - United States President Barack Obama insisted on Friday that Brexit would not harm transatlantic unity but warned against a bitter divorce undermining security in the face of a resurgent Russia.
Britain's shock vote to leave the European Union dominated Obama's final NATO summit, which comes at what he called the most critical time for the military alliance since the Cold War.
Obama used the Warsaw summit to issue a clear message to key US allies Brussels and London to resolve their differences amicably.
"No-one has an interest in protracted, adversarial negotiations," said Obama, who himself warned ahead of the vote that a non-EU Britain would be at the "back of the queue" for trade deals.
But he said that while the British vote had "created uncertainty" about European integration, fears that it could destabilise the relationship between Europe and the United States were exaggerated.
"This kind of hyperbole is misplaced," he said after meeting European Council head Donald Tusk and European Commission chief Jean-Claude Juncker.
Obama said earlier in the Financial Times he was "confident" Britain and the EU could make an "orderly transition to a new relationship". Britain would "continue to be a major contributor to European security", he predicted.
British Prime Minister David Cameron - who was also at his last NATO summit since standing down after the Brexit vote - insisted Britain would not play a "lesser role in the world".
"We are not turning our back on NATO," said Cameron, whose nuclear-armed nation is one of Europe's biggest contributors to the alliance.
NATO chief Jens Stoltenberg added that Brexit "will not change UK's leading position in NATO".
Stoltenberg, Tusk and Juncker later signed a NATO-EU cooperation accord, laying out how the alliance can work with the EU.
Russia was meant to be the focus of the two-day meeting, with NATO endorsing its biggest revamp since the end of the Cold War in response to Russia's 2014 intervention in Ukraine.
The summit is being held symbolically in the birthplace of the Soviet-era Warsaw Pact, NATO's old adversary, and leaders will dine on Friday night in the ballroom where the pact was signed in 1955.
The summit's centrepiece is a "Readiness Action Plan" to bolster NATO's nervous eastern flank in the face of a Russia under President Vladimir Putin that the allies now see as more aggressive and unpredictable.
NATO leaders accordingly approved four rotating battalions in Poland, Estonia, Latvia and Lithuania, up to 4,000 troops in all, as a collective tripwire against fresh Russian adventurism in its old stomping ground, even if not by itself enough to defend the whole theatre.
Obama said 1,000 US troops would be stationed in Poland and Britain said it would deploy 650 troops, most of them in Estonia.
Stoltenberg said Canada would be the lead nation for the battalion in Latvia and Germany in Lithuania.
The plan also included a pledge to spend 2 per cent of annual economic output on defence, ending years of cuts, and the creation of a 5,000-strong "Spearhead" force ready to deploy within days.
Stoltenberg echoed calls by German Chancellor Angela Merkel for "meaningful" dialogue with Russia, with NATO and Russian ambassadors set to meet next week.
"NATO does not want a new Cold War. The Cold War is history and should remain history," former Norwegian premier Stoltenberg said.
But fresh cracks appeared in NATO's unified front on Russia when French President Francois Hollande insisted that Russia was a "not a threat" but a "partner".
Moscow bitterly opposes NATO's expansion into its Soviet-era satellites, which it sees as a threat to its own security.
"We want to believe that common sense and political will to avoid a confrontation will carry the day. Russia remains open for dialogue," Kremlin spokesman Dmitri Peskov said on Friday.
Russia is even more strident in its opposition to the Ballistic Missile Defence system the United States is building.
In a move likely to draw a sharp response, Stoltenberg said leaders had approved putting the system under NATO control after it reached reached an initial operating level.
Washington says the shield is designed to counter missile threats from Iran or the Middle East but Russia says that once the system becomes fully operational in 2018, it will undercut its strategic nuclear deterrent.