Tens of thousands of people in the Darfur region in the east have already crossed the Chadian border. Others are trying to reach South Sudan, the Central African Republic, Egypt or Ethiopia.

The UN's International Organization for Migration has counted more than 3,500 people who have fled the country across the Ethiopian border in the past two weeks. "We are engaged "in a race against time," said Jérôme Merlin, deputy representative of the UN refugee agency (UNHCR) in Chad.