Almost five million people rely on aid in Sudan's Darfur region
More than a million people in Darfur will go without food rations by May unless new aid agencies are deployed, a joint Sudanese-UN assessment says.
It also says there could be major water shortages within two weeks.
The warning follows Sudan's expulsion of 13 large foreign aid agencies, mostly from Darfur.
Mr Bashir accuses them of spying for the International Criminal Court, which has issued an arrest warrant against him for alleged war crimes in Darfur.
Four of the expelled non-governmental organisations (NGOs) served some 1.1 million people, the report released on Tuesday said.
The assessment team toured Darfur from 11-19 March, and the report was co-signed by UN and Sudanese officials.
UN humanitarian affairs coordinator Ameerah Haq told journalists in the Sudanese capital, Khartoum that "the most critical needs are being filled for now".
"However, by the beginning of May, as the hunger gap approaches, and unless the World Food Programme has found partners able to take on the mammoth distribution task, these people will not receive their rations," she said.
The UN's John Holmes on the aid crisis
The assessment also warned that "major water shortages could develop within two to four weeks, as from March 18, if fuel, incentives and spare parts are not continuously provided."
Since the expulsion of aid agencies, Sudan has said Sudanese groups have been filling the gaps, denying that there is any problem with the distribution of aid.
But UN humanitarian head John Holmes said the Sudanese government had not done enough, and that it had agreed in the report that gaps existed.
"We and the NGOs that are left, and the government, can do band-aid solutions, can make sure there is fuel available this week, maybe provide a consignment of chlorine tablets to purify water in some places," he said.
"But to replace the capacity that's gone properly will take time, is difficult, the capacity doesn't exist on the ground at the moment."
The ICC accuses Mr Bashir of orchestrating atrocities against civilians in Darfur, where his Arab-led government has been battling black African rebels since 2003.
Up to 300,000 people have been killed and 2.7 million have been driven from their homes.
Sudan denies the charges and says the figures are exaggerated.
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