ATHENS - Macedonian police used tear gas to push back hundreds of migrants from a border fence on Sunday at a sprawling refugee camp on the Greek side of the frontier, a Reuters witness said.Tear gas was fired on a crowd of more than 500 people who had gathered at the fence at the makeshift camp of Idomeni. A Greek police source said there was tension in the area but declined to comment further.A Macedonian official who asked to remain anonymous said that a large group of migrants left Idomeni camp in the morning and stormed towards the fence."They threw rocks at the Macedonian police. The police fired tear gas in response," the official said."The migrants were pushing against the fence but standing on the Greek side of the border. The fence is still there, they have not broken through."More than 10,000 migrants and refugees have been living at the sprawling tent camp in Greece since February, stranded there after a cascade of border shutdowns throughout the Balkans.Migrants at Idomeni are demanding that the border with Macedonia be opened, but no migrants have been allowed through for weeks.Greek authorities have been trying to convince the population to move to reception camps, with little success.More than a million people fleeing conflict poured into Europe mainly through Greece in the past year. The European Union is implementing an accord under which all new arrivals to Greece will be sent back to Turkey if they don't meet asylum criteria.