Less than a week after the Sudanese government agreed to allow U.N. relief agencies into Darfur province to care for more than a million homeless civilians, Khartoum has raised new obstacles that could delay the delivery of lifesaving supplies to the area for months, according to a senior U.N. official.

The United Nations' emergency relief coordinator, Jan Egeland, said that Sudanese officials told the U.N. Children's Fund that all medical supplies shipped into the country had to be tested at Sudanese laboratories. He also said Khartoum has insisted that food and other relief supplies be transported on Sudanese trucks and distributed exclusively by Sudanese charities or government agencies.

"Given the almost nonexistent capacity of Sudanese [nongovernmental organizations] like the Sudanese Red Crescent Society, the enforcement of this restriction would have a devastating impact," Egeland told the 15-nation council on Wednesday. "If the government wants to prevent a famine from occurring, these restrictions on international NGOs and non-Sudanese trucks have to be lifted without delay."

The unfolding humanitarian crisis in Darfur has eclipsed one of the Bush administration's most important foreign policy achievements in Africa: brokering an agreement to end a 21-year civil war in Sudan that has left more than 2 million dead.

Khartoum's Muslim government and the rebel Sudan Liberation Army, based in the country's Christian and animist south, initialed a landmark revenue-and-power-sharing pact in Naivasha, Kenya, on Wednesday. The move is the first step toward signing a comprehensive peace accord, possibly at a White House ceremony.

That agreement, however, does not address the conflict that has led to the crisis in Darfur, a western province the size of France. The latest round of violence there began in February, when two rebel groups, the Sudan Liberation Army and the Justice and Equality Movement, took up arms. The government responded by organizing Arab militias called the Janjaweed and launching an ethnic cleansing campaign against civilians suspected of supporting the rebels.

Charles R. Snyder, the acting U.S. assistant secretary of state for African affairs, told reporters in Washington on Thursday that the United States would press the Sudanese to renew a cease-fire with the Darfurian rebels. "Ethnic cleansing cannot be allowed to stand," he said. "We can't get the new Sudan we want with Darfur on fire and bleeding the way it is."

"The first thing they have to do is change this humanitarian access problem," he said.

The Janjaweed have killed tens of thousands of civilians and driven more than a million people from their homes, according to human rights investigators and U.N. officials. An additional 130,000 civilians have fled to refugee camps in neighboring Chad.

U.S. and U.N. officials have warned that hundreds of thousands will die from disease or starvation in Darfur unless international relief agencies are granted full access to the region.

After months of inaction, the Security Council on Tuesday adopted a U.S.-sponsored statement expressing "grave concern" and warning that "hundreds of thousands of people are at risk of dying in the coming months."

Facing intense international criticism, the Sudanese government agreed on May 20 to a request by U.N. Secretary General Kofi Annan to speed up the processing of visas and drop permit requirements for travel to Darfur.

Sudan's U. N. ambassador, Elfatih Erwa, said Thursday that his government has been striving to meet the United Nations' request for greater access, including the provision of visas. But he said the government would require more information about the humanitarian organizations that would distribute U.N. aid.

But Egeland told the council this week that Sudan has continued to harass U.N. officials and block access to international charities seeking to distribute aid. Egeland said that one U.N. employee was recently threatened with expulsion "if he raised even once any concerns related to the protection of civilians."

Despite the disruption, Egeland said, the United Nations is mounting a relief operation that will feed more than 500,000 displaced civilians in North and South Darfur. But he warned that heavy rains have cut off access to West Darfur, where more than 400,000 displaced civilians continue to face attacks from the militias. He also said international aid donors have provided only $50 million of the $175 million the United Nations needs to address the crisis.

The U.N. criticism of Sudan comes a week after a senior U.S. aid official accused senior U.N. officials of failing to speak out "more clearly and forcefully" about Sudan's refusal to give relief workers greater access to the region. "The U.N. has a responsibility to speak on behalf of the victims," Roger P. Winter, an assistant administrator at the U.S. Agency for International Development, wrote Egeland in a letter May 21.

Winter also accused U.N. relief agencies, including the U.N. Children's Fund and the World Health Organization, of mismanaging health programs in South Darfur. "Erratic food distributions, inadequate and poorly monitored medical supply programs, and laggard efforts to meet critical water and sanitation requirements of displaced communities are the norm rather than the exception," he wrote.

Egeland said that some of Winter's criticism was "valid" but that some of it was "exaggerated." The United Nations, he said, has quickly organized a "massive logistical operation" with few resources and significant government obstacles. "This has been our number one priority now for at least two months," he said.