A senior US State Department official said on Wednesday the Sudanese government did not see the gravity of the crisis, which the United Nations calls the worst in the world.
"They are in a state of denial. They are in a state of avoidance. They are trying to obfuscate and avoid any consequences," said the official, who asked not to be named.
Secretary Colin Powell, on a visit to Darfur, called for an end to the militias blamed for the crisis when he visited some of the one million people displaced by the conflict.
"We all want them to return to their homes and that will require the re-establishment throughout Darfur of security, the end of fighting, the end of the Janjaweed," Powell told aid workers and Darfuris living in the Abu Shuk camp.
The Janjaweed are militias who have driven tribal villagers off the land in a campaign of ethnic cleansing which human rights groups say is verging on attempted genocide.
For most of his flight to Darfur, Powell held talks with Sudanese Foreign Minister Mustafa Usman Ismail on how to deal with the crisis. In Khartoum on Tuesday evening Powell met Sudanese President Umar Hasan al-Bashir.
Many of the displaced people in
Darfur live in dire conditions
Bashir has promised to disarm the Janjaweed and give the relief organisations access to the region.
But the senior US official said: "He [Bashir] has said these things before. We'll have to see what they actually do."
The US has an interest in stabilising western Sudan since the country is gaining in importance as an oil exporter. It backed a series of peace accords signed by the government and the rebel Southern People's Liberation Army to end a civil war that had claimed hundreds of thousands of lives.
Powell was mobbed by thousands of desperate Sudanese as he toured the Abu Shuk camp which shelters some 40,000 people.
Meanwhile, at the main Khartoum University campus, 22 students were injured in clashes with riot police after a group of students tried to deliver a letter to UN offices appealing for direct United Nations intervention in Darfur.
"When they got about 300m from the UN building, police intercepted them, began hitting them, firing at them, and they released teargas," said Wail Taha, a member of the Khartoum University student union.
Later, as Powell left for Darfur, UN Secretary-General Kofi Annan arrived in Khartoum on a similar mission. The two leaders have warned Sudan of UN Security Council action.
More than 10,000 people have died in Darfur and more than a million been driven from their homes since the revolt against the Arab-dominated government in Khartoum broke out among indigenous ethnic minorities in February 2003.
Many of the displaced persons live in dire conditions, afraid to venture outside the camps or return home for fear they will be killed by the Janjaweed.