Sudanese diplomats have dismissed as "fabrications", reports that hundreds of people were forcibly evicted from a camp near Nyala in southern Darfur.
More than 2m people have fled their homes since the conflict began
Sudan's UN envoy, Abdelmahmood Abdelhaleem Mohamed, told the BBC that eyewitness accounts were "irrelevant, unfortunate and unconfirmed".
An un-named UN official on Sunday saw the forced relocation of refugees at gunpoint from Otash camp to Amakassara.
The UN says this "dangerous precedent" could jeopardise Darfur peace talks.
Some 200,000 people are estimated to have died and more than two million displaced during the four-year war.
UN emergency relief coordinator Sir John Holmes said a UN official witnessed Sudanese security forces with sticks and rubber hoses coercing hundreds of refugees, including women and children, to leave Otash refugee camp on the outskirts of Nyala.
Other witnesses told the BBC they saw 10 vehicles with heavy machine guns surrounding people, while eight trucks were loaded with their belongings.
The refugees have been moved into an area where the UN says it is known that the Janjaweed militia operate.
"We are making a fuss because... this is a very dangerous precedent in an area where there are very many thousands of people in camps, where there are security problems," Sir John told the BBC.
He said the UN wanted to send a message to Khartoum that this was not acceptable behaviour and must not happen again.
'No go areas'
Sudanese diplomats contacted by the BBC rejected the reports.
Mr Abdelmahmood said the allegations were "more than fabrications" aimed at "distracting attention from the Sudanese government's announcement of a unilateral ceasefire to accompany the peace talks".
Asked if he was suggesting the UN emergency relief coordinator would make up the reports, Mr Abdelmahmood said: "We do not want to question his credentials but the way he... the timing leaves a lot to be desired."
And in London, Ambassador Khalid al Mubarak said it was for the Sudanese government to make an official statement "after they make their own investigations on the ground".
He said there were armed men in some of the refugee camps which have become "no go areas" for the authorities and aid workers alike.
The governor of South Darfur told the UN it is his intention to close the camps around Nyala, which are home to as many as 90,000 people.
Otash camp alone has an estimated 60,000 refugees, swollen by numbers of people fleeing violence at the Kalma camp, Darfur's largest, a week ago.
Darfur peace talks
This renewed tension comes as international mediators struggle to save peace talks which opened at the weekend in the Libyan town of Sirte with none of the key rebel groups present.
Organisers are striving to avoid defeatism, and are sending African Union and UN envoys to meet absent rebel groups to try to persuade them to get on board.
Leaders of the two main rebel forces - the Justice and Equality Movement (Jem) and the faction of the Sudan Liberation Movement headed by Abdul Wahid el Nur - have called for the talks to be cancelled for the time being.