ATHENS, June 18 : United Nations Secretary General Ban Ki-moon called on Saturday for countries to do more to help cope with Europe's migrant crisis, saying Greece could not manage on its own. "Greece should not be left alone to address this challenge on its own," Ban told reporters after meeting Prime Minister Alexis Tsipras. "We must work together to protect people and address the causes of displacement. I continue to call for a greatest sharing of this responsibility across Europe and indeed across the world." ?REUTERS