The U.N. human rights office will examine whether Sudan's decision to expel aid groups constitutes a breach of basic human rights and possibly a war crime, a spokesman said Friday.

Rupert Colville said the Sudanese decision to expel relief workers from 13 of the largest aid groups constitutes a "grievous dereliction" of duty, putting the lives of thousands at risk.

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The World Health Organization said the loss of the aid agencies would tear a hole in the body's disease monitoring efforts that could lead to outbreaks of infectious diseases going unchecked.

The U.N. refugee agency said refugee camps in neighboring Chad were ill-prepared to deal with an influx of people crossing the border from Sudan in search of help.

Sudan ordered the organizations out after the International Criminal Court issued an arrest warrant for President Omar al-Bashir for war crimes and crimes against humanity in the Darfur conflict. It has accused the groups such as CARE and Save the Children of cooperating with the court and giving false testimony. The groups deny the accusations.

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"To knowingly and deliberately deprive such a huge group of civilians of means to survive is a deplorable act," said Colville, who speaks for U.N. human rights chief Navi Pillay. "Humanitarian assistance has nothing to do with the ICC proceedings. To punish civilians because of a decision by the ICC is a grievous dereliction of the government's duty to protect its own people."

"This decision by the government could threaten the lives of thousands of civilians," living in camps in Darfur and elsewhere, he added.

World Health Organization spokeswoman Fadela Chaib said the expelled aid groups had been carrying out surveillance of infectious diseases in the region.

A Look at Sudan's Darfur Region

"If they are not helping us do this very vital work, we may see the emergence of infectious diseases," she said.

There is currently an outbreak of meningitis in Nyala, the capital of South Darfur, she said. One of the groups, Medecins Sans Frontieres-Holland, was carrying out meningitis vaccinations in the area before it was expelled.

On Thursday, U.N. Secretary-General Ban Ki-moon said Sudan's decision will cause "irrevocable damage" to humanitarian operations in Darfur and called on the government to urgently reconsider its decision.

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At least 2.7 million people in the large, arid region of western Sudan have been driven from their homes in the war between Darfur rebels and the government since 2003. Ban said 4.7 million people in Darfur are receiving aid.

The U.N. has identified the NGOs expelled as Oxfam GB, CARE International, MSF-Holland, MSF-France, Mercy Corps, Save the Children Fund-UK, Save the Children Fund-US, the Norwegian Refugee Council, the International Rescue Committee, Action Contre La Faim, Solidarites, CHF International and PADCO.

Sudan's expulsion order removes 40 percent of the aid workers in Darfur, roughly 6,500 national and international staff, said Catherine Bragg, the U.N.'s deputy emergency relief coordinator. She said at U.N. headquarters that 76 NGOs had been operating in Darfur along with all major U.N. agencies.

The U.N. humanitarian coordination office says the global body will have a hard time making up for the loss of its aid partners.

"The U.N. is looking into contingency planning to fill the gaps left by the expulsion, but it will be very, very challenging for both remaining humanitarian organizations and the government of Sudan to fill this gap," said spokeswoman Elisabeth Byrs.

"Some of us don't see how these gaps can be fully covered," she added.

Christophe Fournier, president of Medecins Sans Frontieres's umbrella group, MSF International, said there was "absolutely no way" the remaining aid workers would be able to meet the needs of the population in Darfur.

Fournier complained that his aid group was caught up in a battle between the government of Sudan and backers of the ICC indictment.

"We are being held hostage -- we and the population of Darfur -- to judicial and political process," he told reporters in Geneva.