"As difficult as it will be, I am confident that the UK and the EU will be able to agree on an orderly transition to a new relationship, as all our countries stay focused on ensuring financial stability and growing the global economy," Obama wrote in a commentary in the Financial Times on the first day of a NATO summit in Warsaw.

"And, while the relationship between the UK and the EU will change, it is worth remembering what will not change. The special relationship between the US and the UK will endure," added Obama.

Britain's vote to leave the EU in a referendum on June 23 has sent shockwaves through the western political system and is a major topic for discussion at the meeting of the 28 NATO leaders in the Polish capital.

Obama is set to meet EU President Donald Tusk and European Commission chief Jean-Claude Juncker on the sidelines of the summit, as well as holding talks with British Prime Minister David Cameron.

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