US President Barack Obama has joined European leaders to discuss security and economic challenges as President-elect Donald Trump prepares to take office.

The meeting in Berlin with the leaders of Germany, Britain, France, Italy and Spain was likely his last in such a setting before he leaves office. The session expands on lengthy talks he held the day before with German chancellor Angela Merkel.

Gathered around a circular table in Mrs Merkel's Chancellery, the leaders exchanged pleasantries but made no substantive remarks as the meeting started.

Since Mr Obama's arrival on Wednesday on his sixth and last trip to Germany as president, he and Mrs Merkel have focused several meetings on issues of globalisation and trans-Atlantic cooperation.

The talks come largely in the context of what the election of Mr Trump will mean to efforts to seek peace in Ukraine and Syria, the strength of the Nato alliance, trade agreements, efforts to fight climate change and other pressing matters.

At a news conference with Mr Obama on Thursday, Mrs Merkel diplomatically said she was approaching the incoming Trump administration "with an open mind".

Around the world, many are looking to her - one of the longest serving leaders of a major world power, leader of Europe's largest economy and one of the biggest US trade partners - for leadership as Mr Obama leaves office.

He seemed to underscore that reality as he effusively thanked her for her "deep friendship."

He said he could not "ask for a steadier or more reliable partner on the world stage", adding that if she chooses to run for a fourth term next year, he would vote for her if he could.

Joining the two leaders are the heads of countries at the centre of many of the European Union's coming challenges.

Prime Minister Theresa May is preparing the UK for negotiations to leave the trade bloc. Spain's prime minister Mariano Rajoy faces economic woes in his country that have contributed to financial instability in the EU.

Italian prime minister Matteo Renzi's already shaky economy has been rocked by tens of thousands of refugees and French president Francois Hollande's socialist government faces a major challenge from the far-right in elections next year.

Mr Trump has applauded the British decision to exit the EU and had meetings with Nigel Farage, leader of Ukip and a key player in the British decision to leave.

For his part, Mr Obama said on Thursday his hope was that the Brexit negotiations be "conducted in a smooth and orderly and transparent fashion and preserve as closely as possible the economic and political and security relationships between the UK and EU".

Still, he noted that he considered the EU "one of the world's great political and economic achievements".

Germany has emphasised that it respects the British decision to leave, but that the United Kingdom cannot cherry-pick what parts of the EU it wants to keep.

On other issues, Mr Obama said he hoped for continuity of US-European relations under Mr Trump, saying "how important it is that we work together".

He said that "continued global leadership on climate in addition to increasing private investment and clean energy is going to be critical".

He added that the US would "continue to stand united with Germany and our Nato allies" in Afghanistan.

On the refugee crisis, he said he had put in place more robust support from Washington and that he was "hoping that continues beyond my administration".

Following his meetings in Berlin, Mr Obama heads to Peru, the final leg of his last foreign tour, for an Asia economic summit.

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