US Secretary of State Colin Powell has said that many people are dying in the "catastrophe" in Sudan's Darfur area.
Some one million people have fled their homes
Mr Powell is due to visit both the capital Khartoum and the war-torn western region next week.
Human rights campaigners say Arab militias are conducting genocide against black Africans.
About 1.2 million people fled their homes and many are living in camps, where food and medicines are running desperately short.
"The situation is so dire that if we were able to do everything we wanted to do tomorrow, there would still be a large loss of life because of the deprivations that people are under now," Mr Powell said.
During his visit, he is expected to press the government to rein in the "Janjaweed" Arab militia.
"We know the militias are being supported by the government and that the government needs to bring those militias under control," State Department spokesman Richard Boucher said.
Mr Powell's two-day visit to Sudan on Tuesday and Wednesday will be the highest-level US official to visit
Sudan since 1978.
US envoy for war crimes Pierre Prosper has said that "genocide" may be taking place in Darfur but this had not been established.
"I can tell you that we see indicators of genocide and there is evidence that points in that direction," he said in testimony before the House of Representatives International Relations Committee.
If the US decides that genocide is taking place, it is legally obliged to take action.
Mr Prosper also said that the US was considering imposing sanctions on those associated with the Janjaweed.
The Darfur conflict has been described as the worst humanitarian situation in the world by the UN.
It says the killings and mass movement of people in the country is due to "a campaign of ethnic cleansing".