Berlin - President Barack Obama used a meeting with key European leaders on Friday to urge them to work with the incoming administration of President-elect Donald Trump and to discuss steps necessary to resolve the conflicts in Syria and eastern Ukraine, the White House said.Obama's meeting 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.Obama didn't speak to reporters before departing for Peru on Air Force One. The White House said Obama thanked his counterparts for their co-operation during his two terms in office. He urged them to work on common challenges with a Trump administration on the "basis of the core values that define the United States and Europe as open democracies."Since Obama's arrival on Wednesday on his sixth and last trip to Germany as president, he and Merkel have focused several meetings on issues of globalisation and trans-Atlantic co-operation. The talks come largely in the context of what the election of 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.The White House said in a summary of the meeting that the leaders expressed grave concern about the humanitarian situation in Aleppo. They agreed that increased attacks by the Syrian regime and its supporters, including Russia and Iran, should be immediately halted. They also are calling for humanitarian access to the city to be restored.On Ukraine, the White House said the leaders agreed that sanctions against Russia must remain in place until it meets its commitments to resolving the conflict.Obama also briefed his counterparts on the progress made in Iraq in liberating Mosul from Islamic State militants. The White House said the leaders agreed on the need for stability after the city is liberated. Obama also encouraged leaders to continue efforts to expand information sharing throughout the European Union to help disrupt terror attacks.