The US Secretary of State Colin Powell has said the killings in Sudan's Darfur region constitute genocide.
More than a million people have fled their homes in Darfur
Speaking before the US Senate Foreign Relations Committee, Mr Powell said the conclusion was based on interviews with refugees who had fled Darfur.
He spoke as the UN Security Council prepared to debate a second resolution threatening Sudan with sanctions.
Up to 50,000 people in Darfur may have died and a million have been made homeless during the conflict.
Mr Powell blamed the government of Sudan and pro-government Arab Janjaweed militias for the killings.
"We concluded that genocide has been committed in Darfur and that the government of Sudan and the Janjaweed bear responsibility and genocide may still be occurring," Mr Powell said.
More than 1m displaced
Up to 50,000 killed
More at risk from disease and starvation
Arab militias accused of ethnic cleansing
Sudan blames rebels for starting conflict
Mr Powell's conclusion is based on evidence collected by state department investigators, who interviewed more than 1,800 refugees.
Their testimonies, Mr Powell said, showed a pattern of violence which was co-ordinated, not random.
Three quarters of them said the Sudanese military had been involved in the violence, working with the Janjaweed.
The Sudanese foreign affairs minister, Najib Abdul Wahab, rejected the accusation of genocide.
He said that neither the European Union nor the African Union had used such strong language to describe events in Darfur.
The BBC's state department correspondent Jill McGivering says the use of the word genocide does not legally oblige the US to act, but it does increase the moral and political pressure.
Ten years ago the UN was accused of failing to stop genocide in Rwanda.
The Sudanese government says it does not believe its allies within the UN will agree to any sanctions.
A previous UN resolution was passed in July, calling for the pro-government Arab Janjaweed militias to be disarmed. The new draft resolution - put forward by Washington - says Sudan has failed to fully comply.
If Khartoum has still not complied by the proposed new deadline, sanctions may be introduced "including with regard to the petroleum sector". Sudan currently produces about 320,000 barrels of oil per day.
The resolution also calls for:
- the expansion of the number and mandate of the current 300 African Union troops in the country
- international over flights in Darfur to monitor what is happening, and an end to Sudanese military flights there
- UN Secretary General Kofi Annan to assess whether acts of genocide have been carried out and identify the perpetrators.
'Not enough aid'
Critics point out that Colin Powell first demanded Khartoum stop the violence at the beginning of July.
Two months later, they say, the government still has not met key demands, yet the US is proposing allowing it another 30 days.
The US House of Representatives had already declared the violence genocide, but the state department has until now argued that the word is a legal definition and that the data had not been available.
Meanwhile, aid agencies including Oxfam, Care International and Save the Children have accused three nations of failing to give enough aid to Darfur.
The agencies criticised Japan, France and Italy for giving only $6m, $9.6m and $10.8m respectively.
"These are some of the richest countries in the world and they have been some of the poorest donors," an Oxfam spokeswoman said.
The US contributed $206m in 2004-5, and the UK gave $94m.
The European Union as a whole is a large donor, but the agencies point out that a UN appeal for $531m to carry out humanitarian work in Darfur in 2004 has raised only $276m.