One former UN official said the head of the UN in Myanmar (Burma) tried to prevent human rights advocates from visiting sensitive Rohingya areas.
More than 500,000 Rohingya have fled an offensive by the military, with many now sheltering in camps in Bangladesh.
The UN in Myanmar "strongly disagreed" with the BBC findings.
In the month since Rohingya Muslims began flowing into Bangladesh, the UN has been at the forefront of the response. It has delivered aid and made robust statements condemning the Burmese authorities.
But sources within the UN and the aid community both in Myanmar and outside have told
the BBC that, in the four years before the current crisis, the head of the United Nations Country Team (UNCT), a Canadian called Renata Lok-Dessallien:- tried to stop human rights activists travelling to Rohingya areas
- attempted to shut down public advocacy on the subject
- isolated staff who tried to warn that ethnic cleansing might be on the way.
One aid worker, Caroline Vandenabeele, had seen the warning signs before. She worked in Rwanda in the run-up to the genocide in late 1993 and early 1994 and says when she first arrived in Myanmar she noticed worrying similarities.
