GENEVA, June 3 (AFP) -- A humanitarian crisis of enormous proportions is now inevitable in western Sudan's Darfur region and up to one million people could die if aid cannot be delivered there swiftly, international officials warned.

We estimate right now if we get relief in, we'll lose a third of a million people, and if we don't the death rates could be dramatically higher, approaching a million people," US Agency for International Development (USAID) chief Andrew Natsios predicted after a high-level UN aid meeting.

More than one million African civilians have been forced to flee their homes because of an onslaught by government-backed Arab militia and Sudanese troops in Darfur over the past year, and atrocities are continuing, the United Nations said.

The United States, European Union, France and the UN warned Khartoum that it must put a stop to atrocities by militia in the strife-torn region, and iron out "severe restrictions" which are still hampering aid deliveries.

Nearly half of the victims are in the westernmost part of Darfur, where aid agencies are struggling to provide help before the impending rainy season.

"This is also the region where the Janjaweed militia is at its strongest and in spite of the ceasefire agreement... the internally displaced report that they are seeing more atrocities, more rape, more pillage, more murder," said UN Under Secretary General for Humanitarian Affairs Jan Egeland.

Another 700,000 to 800,000 more people in Darfur are likely to run out of what they need to survive within months, the UN added.

Some 150,000 Sudanese refugees have fled across the border to Chad, 50,000 more than previously estimated.

"We admit we are late. Constraints have been so great, some agencies have been so slow, some donors have been so slow, the government restrictions have been so many," Egeland said.

"And the Janjaweed militia have been so harsh on the populations that we will have a humanitarian crisis of enormous proportions even in the best of circumstances," he warned.

The UN said it faced a funding gap of about 236 million dollars for aid in the region until the end of the year.

At the meeting, the United States pledged 188 million dollars over 18 months and the European Union's Commission said it would come up with 10 million more euros, while France promised another 1.4 million euros for refugees in Chad.

Officials were adamant that the pressure was firmly on Sudan's government, amid the "most violent, mean-spirited kind of human conduct imaginable" in Darfur, said World Food Programme (WFP) chief James Morris.

Representing the European Union, Ireland's Minister for Development, Tom Kitt, said: "We must also send a strong unequivocal message to the Sudanese government that it live up to its obligations to protect its citizens and, in accordance with the ceasefire agreement, disarm the militia and give access".

The meeting in Geneva brought together donors, Sudanese and Chad officials, Darfur rebel groups, the United Nations and aid agencies.

"Humanitarian aid is urgent but it is not enough. A political solution is necessary: the Sudanese government's ethnic cleansing must not stand," Kenneth Roth, head of the advocacy group Human Rights Watch Roth said here.

Six human rights monitors were due to be deployed in an area equivalent to the size of France, the UN announced at the meeting.

They will join African Union monitors who are due to oversee a frequently broken ceasefire agreed in April.

On Tuesday, one of two rebel groups -- the Justice and Equality Movement
-- said 24 people had been killed in a two-day assault by government forces in the west Darfur village of Adjidji.

A UN human rights report released last month accused the Sudanese government of committing massive human rights violations in Darfur that may amount to crimes against humanity.