One UN official said the process "appeared to have been well planned". The official continued: "This movement is very large. We have not seen such numbers come into west Darfur before."
The UN refugee agency, UNHCR, sent a team to the border with Chad at the end of May to interview the new arrivals. Fighting in eastern Chad has been steadily increasing and it was thought that many could be refugees. But only a very small number have required support from UNHCR.
"Most have been relocated by Sudanese Arabs to former villages of IDPs (internally displaced people) and more or less invited to stay there," said the UN official.
The arrivals have been issued with official Sudanese identity cards and awarded citizenship, and analysts say that by encouraging Arabs from Chad, Niger and other parts of Sudan to move to Darfur the Sudanese government is making it "virtually impossible" for displaced people to return home.
James Smith, chief executive of the Aegis Trust, said the revelations proved that the Sudanese government was "cynically trying to change the demographics of the whole region", adding: "If the ethnic cleansing has been consolidated because the land has been repopulated it will become irreversible. The peace process will fall to pieces."