The Sudan Government is preventing food and medical supplies from reaching Darfur province, where there is a rebellion, a United Nations envoy says.
Some 500,000 people have fled the fighting in Darfur
Mukesh Kapila said while this could be for security reasons, it could also be to mask alleged human rights abuses.
Darfur rebels have accused the army of killing 25 people in an air raid.
Some analysts fear that the fighting in Darfur could undermine the progress being made to end the 20-year war in the south.
Some 500,000 people have fled the fighting in Darfur and the International Committee of the Red Cross has launched an appeal for $1.8m to help some of the worst affected.
"One must say there is a prima facie case that some of the denials of access may well be related to the discomfort of the parties concerned to allow international witnesses," Mr Kapila, the UN's Humanitarian Coordinator in Sudan, told the UN's Irin news agency.
Diplomats have described the fighting in Darfur as "ethnic cleansing" with Arab militias, possibly backed by the government, destroying entire villages inhabited by dark-skinned people who speak African languages.
An official from the Justice and Equality Movement (Jem) accused the government of targeting civilians during an air raid on Wednesday.
"Civilians had escaped an area where there was a battle... and the government plane followed them and bombed them," said Abu Bakr Hamid al-Nur.
The government has not commented on the allegations but a local governor denied being linked to the Arab militias.
Jem has not signed a truce with the government, unlike the other Darfur-based rebels, the Sudan Liberation Army.
John Prendergast, a special adviser with the think-tank International Crisis Group, said the government had armed Arab militias at the same time it was trying to resolve the war in the south.
"The government of Sudan is being feted by the international community for its transition to peace-maker through the... process (in Kenya), while it continues to carry out a bloody campaign by proxy against the people of Darfur," he said.
"The end of one tragic civil war in Sudan should not be allowed to be a catalyst for a new one."