A strong and unified Europe is "good" for the United States and NATO is "absolutely vital" for US security and prosperity, US President Barack Obama said Tuesday as he launched his final European trip.

"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."