Emergency relief coordinator Jan Egeland said although UN groups had access, non-governmental organisations were facing slow bureaucratic and procedural obstacles.
"Some ministers are helping us, but some of their subordinates are sabotaging us," he said after briefing the UN Security Council on civilians in war zones on Monday.
Medecins Sans Frontieres, an international medical relief organisation, is experiencing undue delays in getting visas and bringing in equipment, medicine and food.
For example, Egeland said, radios needed for emergency communications were stripped from vehicles because Sudanese authorities believed they were a security liability.
"If they have no radio, they cannot go into Darfur," he said. "They see this as a security risk for the government. We see it as a security necessity for us."
The UN estimates fighting in Darfur has affected more than two million people.
More than half have been driven from their homes, with 130,000 fleeing into neighbouring Chad to escape militias that have allegedly killed, tortured and
"Some ministers are helping us, but some of their subordinates are sabotaging us"
emergency relief coordinator
raped African villagers.
Egeland said, "We are still seeing grown men attacking defenseless woman and children with their automatic rifles," he said.
Some UN officials have accused Sudanese troops of acting in concert with the militia, a charge Khartoum has denied.
Egeland estimated that 800,000 people would receive food rations by the end of the month and this figure would increase to 2 million by October.
But he feared epidemics would break out because water was scarce, wells were dry and health facilities were scarce.
Medecins Sans Frontieres said it still had 50 pending visa requests from May and had medical supplies impounded when they arrived by sea and not by emergency air transport.
Egeland said Sudan was not alone in preventing access to conflict zones, which had left 10 million people in 20 nations without basic necessities.
He also named the Central Africa Republic, the Democratic Republic of the Congo, Ivory Coast, Afghanistan, the northern Caucuses and northern Uganda as other offenders.
However, Egeland described the humanitarian situation in eastern Democratic Republic of Congo as the worst in the world today.
The situation has worsened drastically since fighting erupted in Bukavu earlier this month.
He said 3.3 million civilians were out of reach of aid agencies - the largest number of any conflict - and that civilians are being targeted.
"The world has not understood how deep the crisis has become and what is at stake," he said.
"Access wise, it is even worse than Darfur in western Sudan, where aid groups recently were permitted to enter."