"We believe a strong, prosperous and unified Europe is not only good for the people of Europe but good for the world, and good for the United States," Obama told Greek President Prokopis Pavlopoulos in Athens.
He added that the trans-Atlantic relationship is "the cornerstone of our mutual security as well as prosperity."
Obama will seek to calm the nerves of allies concerned by Donald Trump's shock presidential election victory, especially as the brash Republican has downplayed the importance of the NATO military alliance.
"Across Democratic and Republican administrations there is a recognition that the NATO alliance is absolutely vital," he said.
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