Nairobi/Geneva - The human rights group Human Rights Watch (HRW) warned on Friday of a possible humanitarian disaster in Darfur, western Sudan, if the Khartoum government went ahead with its threat to expel aid organizations.

The threat by President Omar al-Bashir to withdraw work permits for 13 Western aid agencies came after the International Criminal Court (ICC) on Wednesday issued a warrant for his arrest on charges of war crimes and crimes against humanity.

That decision puts at risk the lives of more than 1 million people, HRW Africa Director Georgette Gagnon warned.

Sudan is obliged under international law to allow aid to people in conflict zones as soon as possible, she pointed out.

In response to the arrest warrant from the ICC - the first the UN court has issued against a sitting head of state - al-Bashir called a rally of thousands of supporters in the capital, and announced the effective banning of a total of 13 aid agencies.

The groups expelled include the International Red Cross, Oxfam (Britain), Doctors Without Borders (France) and CARE (US).

Ten were banned on Wednesday with a further three having their work permits revoked on Friday. According to a government spokesman in Khartoum, more aid organizations will now be subject to "review."

Hasabu Mohamed Abdel-Rahman, head of the Commission for Humanitarian Affairs in Khartoum, accused the aid groups of "cooperating" with the ICC.

Human Rights Watch's warning was echoed in Geneva by UN officials and Medicins Sans Frontiers (MSF).

"More than 1 million people are relying, on a daily basis, on international aid groups for food, water and other basic needs," Christophe Fournier, head of MSF International told reporters.

MSF's presence in Darfur was "vital," he said.

"We have treated more than 200,000 people in our clinics and hospitalised more than 8,000 patients," said Filipe Ribeiro, the director of MSF-France, one of the expelled groups. "20,000 pregnant women in 2008 got medical assistance from us."

"We are leaving these people behind," he added.

Some of MSF's offices in Sudan were raided by the police and officials from the government, according to Ribeiro. They took computers, cars and medicines, he said.

Khartoum has justified its actions in banning the aid agencies by accusing them of being allied with The Hague-based ICC - a charge denied by the non-governmental organisations.

"We have obviously nothing to do with the ICC. We have said this repeatedly. We do not cooperate or give any information to the ICC," Fournier said. "This is well known, including by the Sudanese authorities."

"We are being held hostage, us and the population of Darfur, to the political process," Fournier said.

The UN Office for the Coordination of Humanitarian Affairs said the government's decision to expel the groups would "have a devastating effect on their own citizens in Darfur."

There is also concern about possible migration of large numbers of already needy and displaced people if the aid agencies leave.

"Our experience shows that when vulnerable populations are unable to the help they need, the go elsewhere in search of protection and assistance," the UN's refugee agency, UNHCR, said.

On Thursday the UN Secretary General, Ban Ki-moon, accused the Sudanese government of inflicting "irrevocable damage" in Darfur by expelling the relief agencies.

The UN said relief groups that operated in Darfur represented a lifeline to 4.7 million Sudanese people in Darfur.

Ban said in a statement that the relief groups had been working to meet the needs of Darfurians "in a neutral and impartial manner."

"As such, I appeal to the government of Sudan to urgently reconsider the above decision," Ban said in a statement.

"The secretary general is also concerned about the safety and security of national and international humanitarian workers in Sudan and their assets," the statement said. "The confiscation of equipment, money and other materials is unacceptable and must end immediately." (dpa)