The United Nations will investigate whether Sudan's decision to expel aid groups in the wake of an arrest warrant being issued for its president constitutes a breach of basic human rights or is a war crime, said its human rights office.

The Sudanese decision to expel relief workers from 13 of the largest aid groups is a "grievous dereliction" putting lives of thousands at risk, UN Human Rights Council spokesman Rupert Colville said Friday.

"To knowingly and deliberately deprive such a huge group of civilians of means to survive is a deplorable act," said Colville, who speaks for UN human rights chief Navi Pillay. "Humanitarian assistance has nothing to do with the [International Criminal Court] 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."

Sudanese President Omar al-Bashir's government ordered the expulsion of 13 international organizations from Darfur following the International Criminal Court issuing a warrant for his arrest for war crimes and crimes against humanity.

The expelled groups include Oxfam, CARE and Save the Children. Their departure removes about 40 per cent of the aid workers, or 6,500 people, from the region.

Seeking options to deal with gaps

Though the UN had taken into account the possibility that al-Bashir might expel aid workers, the order for them to leave immediately came as a surprise, said UN deputy emergency relief co-ordinator Catherine Bragg.

"The UN 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 UN humanitarian co-ordination spokeswoman Elisabeth Byrs.

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

Al-Bashir accused the organizations of disrupting peace efforts in the region and alleged they have violated the law and security of Sudan.

The groups have denied the accusations.

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

Ban said 4.7 million people in Darfur are receiving aid. The absence of the organizations in the country could heighten the crisis for the Sudanese, who still need shelter, food and clean water, he said.

Camps not ready for influx

The loss of the aid agencies will have implications for the management of disease in the country and could result in infectious outbreaks going unchecked, the World Health Organization said Friday.

People from Sudan are crossing the border into neighbouring Chad, said the UN refugee agency, but camps in the region weren't prepared to deal with the influx.

The warrant for al-Bashir marks the first time the ICC has issued one for the arrest of a head of state who is still in power. The judges in The Hague say the Sudanese president is alleged to have committed war crimes and crimes against humanity during a five-year anti-insurgency campaign.

That campaign, it is alleged, was waged against the Sudanese Liberation Movement Army, the Justice and Equality Movement and other armed groups who complained of decades of neglect and discrimination by the Arab-dominated government.

The fighting, which began in 2003, has led to the deaths of more than 300,000 people and forced nearly 2.7 million others from their homes.

Meanwhile, Iran and Hamas and other militant Palestinian groups have sent delegations to Sudan to show support for al-Bashir.

Supporters of al-Bashir were also marching in the streets of Khartoum on Friday and offering prayers for the president.

With files from the Associated Press