A catastrophe is now unavoidable in Sudan's Darfur region, the United Nations and aid workers say.
The UN fears hundreds of thousands will die in Darfur
Some 300,000 people will starve, even if emergency aid is delivered immediately, according to the head of the United States aid agency.
Some 10,000 people have died, and a million made homeless in a conflict between rebels and Arab militias.
UN officials blame Sudan's government, which they say supports the militias as they rape and kill Darfur's people.
"If we get relief in, we could lose a third of a million. If we do not, it could be a million," said Andrew Natsios, head of USAid.
The figures were based on mortality and malnutrition rates, he said.
"But that is not a prediction, and we hope it is not true."
He was speaking at a donor conference, where the UN appealed for $236m for Darfur.
The US pledged $188m over 18 months and the EU 10m euros.
The rainy season is about to start in Darfur, and roads will turn to quagmire.
Aid agencies say they will have to resort to air drops to get supplies through - a desperate measure which is rarely very successful.
Jan Egeland, Under-Secretary General for Humanitarian Affairs, said that despite the recent ceasefire, the violence was continuing.
"These are totally defenceless people," he said. "Women and children for the most part, and those who kill them are grown men with Kalashnikov automatic rifles."
Food and water are scarce, disease is spreading, crops have failed and famine now threatens, the UN officials said.
In some refugee camps, the infant mortality rate is already 25 times the international average.
More than 100,000 people have fled to neighbouring Chad.
Representatives of the Sudanese government and rebel groups attended the meeting, to try to address concerns that aid agencies are being prevented from reaching those most in need.
But UN officials urged the sides to control militias active in the area.
"Our appeal is to the government to get its act together and to get those people under control quickly," World Food Programme head James Morris said.
Mr Egeland said that the international community had been too slow to react to the crisis.
"We admit we are late," he said. "Constraints have been so great, some agencies have been so slow, some donors have been so slow, the government restrictions have been so many."