[Migrants stand next to the Greek-Macedonian border fence, near the Greek village of Idomenii]By Stephanie Nebehay and Gabriela Baczynska GENEVA/BRUSSELS (Reuters) - The build-up of thousands of migrants and refugees on Greece's northern borders is fast turning into a humanitarian disaster, the United Nations said on Tuesday as the European Union prepared to offer more financial aid. German Chancellor Angela Merkel said clashes at Greece's border with Macedonia on Monday - when migrants battered down a gate and were tear-gassed - simply underlined the urgency with which the EU needed to act on the crisis.