More than 100 women have been raped in a single attack carried out by Arab militias in Darfur in western Sudan.
Aid agencies cannot get help to thousands of displaced people
Speaking to the BBC, the United Nations co-ordinator for Sudan, Mukesh Kapila, said the conflict had created the worst humanitarian situation in the world.
He said more than one million people were being affected by ethnic cleansing.
He said the fighting was characterised by a scorched-earth policy and was comparable in character, if not in scale, to the 1994 Rwandan genocide.
"It is more than just a conflict. It is an organised attempt to do away with a group of people," he said.
Arab militias, backed by the government, have driven hundreds of thousands from their homes, in retaliation for a rebellion launched a year ago by two armed groups.
They accused the Arab-dominated government of ignoring the black African inhabitants of Darfur.
More than 100,000 people have fled across the border into Chad, but have continued to face cross-border raids.
Mr Kapila said 75 people were killed in the attack on the village of Tawila at sunrise by Arab militiamen two weeks ago.
"All houses as well as a market and a health centre were completely looted and the market burnt. Over 100 women were raped, six in front of their fathers who were later killed," he said.
A further 150 women and 200 children were abducted.
This attack a fortnight ago is one of many across the arid territory.
Village after village is being razed to the ground by the militias, he said.
To compound the problem, aid agencies can only reach small parts of Darfur and are subject to attacks.
Mr Kapila called for more aid and for urgent international intervention to bring about a ceasefire in the war.
"I was present in Rwanda at the time of the genocide, and I've seen many other situations around the world and I am totally shocked at what is going on in Darfur," he told the BBC Radio 4's Today programme.
"This is ethnic cleansing, this is the world's greatest humanitarian crisis, and I don't know why the world isn't doing more about it."
The fighting in the west of Sudan has intensified as government peace talks to resolve the 20-year war with southern rebels are nearing an end.
But the UN is concerned that this conflict could undermine the peace talks in Kenya.