The UN humanitarian chief said Monday that the world's largest aid effort was in danger of "unraveling with catastrophic effects" if access to refugees in Darfur were not improved and steps taken to end the violence there. John Holmes, ending his first visit to Darfur as the UN's top official for humanitarian relief, said problems including continued violence and Sudanese government obstruction were endangering international relief operations. "If we don't deal with those problems, the humanitarian effort could begin to unravel with catastrophic effects for the civilians," he said. That would affect some four million people in the Darfur region who are caught in fighting between rebels, government forces and pro-government militias. Ethnic African civilians have suffered most at the hands of a predominantly Arab government and its janjaweed militias.