UNITED NATIONS - UN agencies and other organizations allowed to remain in Darfur don't have the resources to fully replace the activities of 13 expelled aid groups who were helping millions in the conflict-wracked region, the UN's humanitarian chief said.

The United Nations will try to fill life-threatening gaps left by Sudan's expulsion of more than half the aid workers from Darfur, John Holmes, the UN undersecretary-general for humanitarian affairs, said Monday.

But neither the world body nor the Sudanese government has the capacity to take over the work of 13 of the largest humanitarian organizations kicked out of the country, he said.

Those organizations were ordered to leave Darfur after the International Criminal Court ordered an arrest warrant for President Omar al-Bashir on charges of war crimes and crimes against humanity in Darfur. Sudan has accused them of cooperating with the Netherlands-based ICC.

The State Deparment on Monday warned U.S. citizens against traveling to Sudan due to the "uncertain" security conditions following the expulsions and said U.S. citizens already in the country should be prepared to leave. Non-emergency personnel and family members at the U.S. Embassy in Khartoum have been authorized to depart the country, according to the travel warning.

The Netherlands-based ICC accuses al-Bashir of leading a counterinsurgency against Darfur rebels that involved rapes, killings and other atrocities against civilians. Up to 300,000 people have died and 2.7 million driven from their homes in the conflict since 2003, according to the UN.

According to the latest UN figures, 7,610 people worked for the 13 aid groups -- 308 international staff members and 7,302 Sudanese, mainly from Darfur, he said. They represent more than 50 per cent of the roughly 14,000 humanitarian workers from 85 organizations that had been working in Darfur, including about 1,000 international staff, he said.

"This is a decision ... which is likely to have a major impact on millions of people in Darfur, in particular, who are in need on a daily basis of lifesaving humanitarian assistance," Holmes said. "That's why the UN opposed the decision so strongly and asked the government of Sudan to reverse it as soon as possible."

Last week, UN Secretary-General Ban Ki-moon said the relief agencies are key to maintaining a lifeline to 4.7 million people in Darfur. Holmes' deputy, Catherine Bragg, added that the loss of the 13 NGOs would mean "1.1 million people will be without food aid, 1.1 million will be without health care and over 1 million will be without potable water."

Holmes said the UN has been asking the Sudanese to justify the expulsions, but has not received a satisfactory explanation.

"I think it's reasonably clear this was a political response to a decision which has got nothing to do with the UN and nothing to do with the NGOs," he said.

While the UN continues to urge the Sudanese government to reverse its expulsion orders, Holmes said it will also start working with the government to find out where the most critical gaps lie.

The Sudanese government's humanitarian agency said it has agreed with UN humanitarian officials to send a "joint technical assessment mission" to evaluate the situation in Darfur following the explusion of the 13 aid groups. The joint team will travel to Darfur on Wednesday, the agency said.

Assets of international NGOs have been confiscated including vehicles and computers, vital data for assistance to beneficiaries and food and non-food items, he said. There also have been cases of intimidation by some Sudanese officials toward NGO staff facing expulsion, he said.

Holmes said one or two warehouses containing food from the UN World Food Program which was going to be distributed by NGOs were also quickly seized by local Sudanese authorities. "We have been assured that these assets will be returned to us quickly," he said.