'A strong and united Europe is a necessity for the world' Obama said in the German city of Hanover in a landmark speech that carried the tone of a blunt challenge to friends.

Visiting a region reeling from a migration crisis economic stagnation and facing the prospect of Britain abandoning the European Union (EU) Obama warned that 'progress is not inevitable'.

Contrasting the prosperity of Europe today with the wars and hardship of the last century Obama called on Europeans reject the 'us-versus-them' politics that has fuelled the rise of the far right in countries from Poland to France.

'Perhaps you need an outsider somebody who is not European to remind you of the magnitude of what you have achieved' he said a day after the anti-immigration far-right triumphed in a presidential vote in Austria.

Obama painted today's Europeans as heirs to the popular movements that ended the Cold War. And he also recalled the devastating consequences of 'intolerance and extreme nationalism' that drenched Europe in blood during the 20th century.

'In the last century just twice in 30 years the forces of empire and intolerance and extreme nationalism consumed this continent and cities like this one were largely reduced to rubble' Obama said.

'Tens of millions of men and women and children were killed.'

While admitting there could be frustrations with European institutions he argued that 'turning inward' was not the answer to Europe's problems.

As rightwing populism gains ground in parts of the continent in response to growing radicalism he urged Europe to remain open.

'I want you to remember that our countries are stronger they're more secure and more successful when we integrate people of all backgrounds and faiths and make them feel as one. And that includes our fellow citizens who are Muslim' he said.

On a visit to Britain ahead of his arrival in Germany the US President had also waded into an increasingly-bitter debate over the UK's membership in the European Union urging Britons to vote against leaving the bloc in a June referendum.

For much of Obama's seven years in the White House his relationship with Europe has been uneasy. Obama began his presidency with Europeans revelling in Washington's more relaxed approach to foreign policy than under his predecessor George W Bush. But since then Obama's star has dimmed and the US President has become frustrated with Europe's inability to move quickly in response to the global recession or to the threat from militants.

In a mark of that frustration Obama bluntly told his audience that Europe needs to do more to shoulder the collective security burden. 'Europe has sometimes been complacent about its own defence' Obama said repeating a long-standing call for NATO allies to increase defence spending to at least two per cent of economic output.

More than half a century after the end of World War II much of the continent including economic power Germany remains firmly under the umbrella of security provided by the United States.