Alun McDonald, the spokesman for the British aid group Oxfam in Sudan, said: "Before, aid workers were being caught up in the violence, now they're actually being targeted.
"It's not the occasional vehicle getting lost, it's compounds being looted in coordinated attacks."
'Trying to understand'
UN and other humanitarian groups say the turning point came on December 18, when at least four different aid organisations were attacked in the South Darfur camp of Gereida, the regions largest with 130,000 refugees.
Offices and housing compounds were simultaneously raided, cars and radio equipment plundered, and some aid workers endured mock executions while the unidentified assailants raped a female aid worker from the French aid group Action Contre la Faim.
Philippe Conraud, ACF's Sudan coordinator, said: "We're still trying to understand what happened.
"The fact that we can't assess why this took place is worrying us for the future."
Nearly all aid workers have evacuated Gereida, a zone reputed to be controlled by fighters from the faction of the Sudan Liberation Movement led by Minni Minawi, the one rebel leader who signed a peace agreement with the government last May.
Aid workers had initially thought the attacks might have been carried out by a rebel squad seeking equipment so that it could break from Minawi and resume fighting, but now say they have no idea who was responsible.
Conraud said Minawi had provided him with "very little explanation", but had promised to fully investigate the incident.
Aid groups and UN officials have for months been alarmed by the rapidly deteriorating conditions faced by civilians in Darfur, where over 200,000 people have been killed since rebels took up arms against the central Sudanese government in 2003, accusing it of neglect.
The government is accused of retaliating against civilians with air raids and by unleashing the janjaweed, militias of Arab nomads blamed for the worst atrocities in the conflict.
The statement said: "Villages have been burnt, looted and arbitrarily bombed and crops and livestock destroyed. Sexual violence against women is occurring at alarming rates. This situation is unacceptable."