[A worker removes tents used by migrants during a police operation to evacuate a migrants' makeshift camp at the Greek-Macedonian border near the village of Idomeni]GENEVA (Reuters) - Refugees in some sites in Greece are cramped in "sub-standard conditions" in poorly ventilated "derelict warehouses and factories", with insufficient food, water, toilets and showers, the United Nations refugee agency said on Friday. "We urge the Greek authorities, with the financial support provided by the European Union, to find better alternatives quickly," UNHCR spokeswoman Melissa Fleming told a briefing. (Reporting by Stephanie Nebehay; Editing by Louise Ireland)