UN Secretary General Kofi Annan has called on the Sudanese government to stop its troops forcing refugees out of camps in the western Darfur region.
The violence has led 1.5 million people to flee their homes
Aid workers said hundreds of internally displaced people had been moved from camps near Nyala by government forces.
Earlier the UN's chief envoy in Sudan, Jan Pronk, said the action was in "flagrant violation of international humanitarian law".
The army said it had been asked to protect refugees and evict impostors.
Mr Annan called on the government to immediately halt the relocation of refugees and return them from the "inappropriate sites" to which they had been taken.
He also said in a statement he was concerned at the deteriorating situation in Darfur - in particular the kidnapping of 18 people by the Sudan Liberation Army (SLA) and the mobilisation of thousands of Arab militias.
"The SLA and the militias risk sparking a new round of violence that could claim the lives of thousands of civilians," he warned.
Police and army troops surrounded the Abu Sharif and Otash camps near the town of Nyala in the early hours of Tuesday morning, the World Food Programme's Barry Came said.
In the case of Abu Sharif camp, the government said their forces had been invited into the camp after local leaders said there were residents from local towns there pretending to be refugees to get food rations.
According to Mr Came, the security forces moved a number of people from Abu Sharif camp elsewhere.
There were no reports of any deaths, he said, but humanitarian agencies had yet to regain access to the camps.
"This is part of an increasing pattern of deteriorating security conditions right across Darfur," he told the BBC's Focus on Africa programme.
The WFP estimates that about 160,000 people cannot be reached by aid workers.
Pro-government Janjaweed militias are accused of driving the region's black African population from their villages, since two rebel groups took up arms against the government in February 2003.
The senior African Union (AU) officer in Nyala, Lieutenant Colonel Henry Mejabi, told the BBC the ceasefire, agreed in April, was being violated every day by the government and both rebel sides in Darfur.
Fighting was reported on Monday near Nyala between the Sudan Liberation Movement (SLM) rebels and government forces.
Travellers arriving in Nyala on Monday afternoon said that they have spotted several bodies lying on the road. Some were wearing uniforms, others the traditional jalabia robe.
Earlier, as a precautionary measure, UN agencies banned their staff from travelling on that road.
A small number of the promised AU peacekeepers are in Darfur
Further on, road blocks set up by Arab militiamen are preventing commercial and humanitarian vehicles from travelling.
Acts of banditry are also on the rise. Civilians have been attacked at roadblocks in the villages and even inside displaced camps.
The head of the AU's peacekeeping operation in Sudan, Said Djinnit, said the organisation now has nearly 250 troops in Darfur.
AU mediators have presented the Sudanese government and rebel groups with a new security deal at talks taking place in the Nigerian capital, Abuja, to end the conflict.
The draft includes a proposal to make Darfur a no-fly zone - a key demand of the rebels.
Spokesmen for each side said the new draft was better than a previous one.