US President Barack Obama said Friday that nobody has an interest in "protracted, adversarial" negotiations on Britain's departure from the European Union, following the country's shock decision to leave the 28-member bloc.

The June 23 referendum vote for Britain to leave the EU has raised concerns about the union's stability and future.

The vote "has created uncertainty about the future of European integration," Obama said at a press conference in Warsaw with EU President Donald Tusk and European Commission President Jean-Claude Juncker.

"This has led some to suggest that the entire edifice of European security and prosperity is crumbling," he added.

But Obama expressed confidence, following his talks with Tusk and Juncker, that Britain and the EU "will work together in a pragmatic and cooperative fashion to ensure that the UK's transition is orderly and smooth."

Negotiations will not take place "in a hostile mood," Juncker added, noting that Britain and the EU would continue to share a "community of interests."

"Maintaining the closest possible relations between the EU and the UK is in European and American interest," Tusk added, while warning that the "geopolitical consequences of Brexit may be very serious."

Obama praised European integration as "one of the greatest political and economic achievements of modern times," noting that Washington has a strong interest in a united and democratic Europe.

At the same time, he stressed the need - on both sides of the Atlantic - to address "economic frustrations and anxieties" of people who fear they are being "left behind by globalization and economic integration."

"Our governments, including the EU, cannot be remote institutions," the US president added.

Obama met Tusk and Juncker in Warsaw on Friday morning, before attending a NATO summit. He is also expected to hold talks with British Prime Minister David Cameron there.

"I have no doubt that the UK will remain one of NATO's most capable members — a nation that pays its full share for our common security and is a leading contributor to alliance missions," Obama wrote earlier in the Financial Times newspaper.

"Given the current threats facing Europe, I fully expect that Britain will continue to be a major contributor to European security," he added. "The special relationship between the US and the UK will endure."

The EU and US leaders also reviewed negotiations on the mammoth and controversial free trade agreement being negotiated between Brussels and Washington, commonly known as TTIP.

"While we are mindful of the challenges, we are going to continue to pursue a Transatlantic Trade and Investment Partnership," Obama said.

"We want to conclude these negotiations before the end of this year, mainly as far as the big blocks of this negotiation are concerned," Juncker added.

Many are questioning whether they will manage to wrap up negotiations before Obama leaves the White House in January, with many key issues unresolved. A new round of negotiations is taking place in Brussels next week.

Tusk and Juncker were seeking "movement from the US on issues important to us," a senior EU official said on condition of anonymity, ahead of their meeting with Obama. No details emerged after the talks, however.

Their discussions, which lasted almost an hour, also touched on joint efforts to combat terrorism, as well as global migration challenges and economic issues.

"Europe will remain a cornerstone of America's engagement with the world," Obama said, noting that the US and the EU have the world's largest trade and investment relationship, while their security is "indivisible."

The trip is Obama's last planned visit to Europe before he leaves office in January.

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