[United Nations High Commissioner for Refugees Filippo Grandi meets with Greek Prime Minister Alexis Tsipras at the Maximos Mansion in Athens]Serious challenges lie ahead in dealing with Europe's migrant crisis, the United Nations' refugee chief said on Wednesday as he urged the European Union to help its frontline member-country Greece manage the flows of people. Greece, struggling to emerge from a severe debt crisis, has been the main gateway to Europe for nearly a million refugees fleeing war and poverty in the Middle East. Thousands of migrants and asylum seekers are trapped in overcrowded camps in the country since an EU-Turkey pact came into force this year.