The United States has accused Sudan of deliberately preventing American relief workers from visiting the troubled western Darfur region.
About a million of Darfur residents are now displaced
A spokesman said that after intense pressure, 11 aid workers had been given three-day permits to visit Darfur.
But he said the permits were rendered useless because three days' notice was required before travelling.
Some one million people have fled Darfur, where pro-government militias are accused of "ethnic cleansing."
"There are people suffering in Darfur. It's urgent that humanitarian workers be allowed to go there," said deputy State Department spokesman Adam Ereli.
"It's Orwellian," a senior State Department official said.
The Sudan government has sent some 1,000 police officers to Darfur after downgrading its security rating.
"The situation in Darfur is stable and the threat indicator has been brought down from the colour red to the colour yellow," said Ahmed Mohamed Haroun, state minister for internal affairs.
Red is the highest of three security levels.
The government has claimed on several previous occasions to have defeated the Darfur rebels, who took up arms last year.
Last week, medical charity Medecins Sans Frontieres (MSF) said the lives of tens of thousands of people are at risk from hunger and disease.
The charity criticised the UN Refugee Agency (UNHCR), for being "painfully slow" in providing relief.
The UNHCR in turn says it has not received the money it has asked for from international donors.
Some 130,000 people have fled to neighbouring Chad, while an estimated 900,000, mostly black Africans, are displaced within Sudan.
They say the Arab "Janjaweed" militia are chasing them from their homes and are working with government forces.
The UN has accused Sudan of tolerating "ethnic cleansing" in Darfur, a charge the government in Khartoum denies.