Sudanese President Omar Hassan al-Bashir says he wants international humanitarian groups out of the country within a year.
His comments follow the expulsion of 16 large aid agencies from Darfur on March 4, an order that left the humanitarian community "in a state of shock", according to an official from Médecins Sans Frontières (MSF) Switzerland.
Tension between Sudan and the international community grew after the International Criminal Court (ICC) earlier this month issued an arrest warrant for Bashir over accusations of war crimes in Darfur.
Sudan reacted to the ICC warrant by shutting down 13 foreign aid organisations and three national agencies, saying they had helped the international court in The Hague, Netherlands.
It is an accusation aid groups deny, although on Monday, the International Rescue Committee, one of the major organisations in the region, admitted it had once considered cooperating with the ICC but said it later rejected the idea.
The United Nations said the expulsion of the 16 agencies, which include Oxfam, Care International, MSF-France and Netherlands, and Save the Children UK, would lead to "irrevocable damage" and risk a humanitarian crisis.
"Agencies on the ground are in a state of shock, trying to recover and get clarity from the authorities about the needs and what's next," said Christian Captier, director general of MSF-Switzerland. The group still has a small team working in western Darfur, alongside MSF teams from Belgium and Spain, in northern Darfur.
"It's currently a very challenging phase. Everything is changing very rapidly. I really don't know what will be the next steps," Captier said.
At a rally of security forces on Monday, Bashir told humanitarian organisations to quit the oil-producing African country, saying Sudan's humanitarian affairs ministry would nationalise the work.
"Within a year, we don't want to see any foreign aid group dealing with a Sudanese citizen," Bashir said. "If they want to bring relief, let them drop it at airports or seaports. Let the national organisations deal with our citizens."
He did not elaborate on how the order would be carried out or what would happen to the 70-plus foreign aid groups inside Sudan and government officials later tried to water down the president's comments, saying it was a "process" and would leave UN agencies untouched.
Switzerland's foreign ministry said it was still trying to confirm the order. There are few Swiss NGOs active in Sudan.
"If in fact all international aid groups were to be expelled from Sudan, the humanitarian consequences would be very significant for a huge number of Sudanese people living in Darfur and the northern parts of Sudan," spokesman Andreas Stauffer said in an email on Monday evening. He said the Swiss government would monitor the situation.
Bashir's comments come two days after three foreign aid workers from MSF-Belgium were released after being kidnapped for several days in Darfur, apparently in retaliation for the ICC arrest warrant.
Earlier this month the Swiss foreign ministry said it "deplored" the expulsion of the 13 aid agencies and said it was "very worried about the humanitarian and human security consequences [the expulsion] could have for the 4.5 million people in distress in Darfur".
"According to the UN, within the next week 1.1 million people will need food, 1.2 million people will need water and 1.5 million people will be without access to health services as a result of the expulsions," Melanie Brookes, spokeswoman for Care International in Geneva, told swissinfo.
The biggest challenge will be transferring programmes to groups remaining in Sudan, she added.
"Thirteen of the largest agencies operating in Sudan were effectively shut down without a moment's notice. All of those programmes have stopped, so there will be a lag time before they can be picked up... or if they can be picked up," said Brookes.
The expelled agencies employed 40 per cent of the staff in Sudan but were providing almost 60 per cent of the humanitarian aid in Darfur, she explained.
Press for reversal
The UN is continuing to press for the reversal of the Sudanese government's decision but there is "little chance" of the government changing its mind, said Captier. "If the Khartoum government doesn't want any more international NGOs we have to accept that, reluctantly, but in the end they are the ones in charge," he said.
At the same time, a joint UN-Sudanese assessment mission to Darfur is assessing the most urgent needs and the gaps left by the expulsions.
Last week Sudan declared it could "fill the gap" by their departure by cooperating with the UN and the International Red Cross and Red Crescent Movement and by mobilising local organisations.
"The ICRC has its largest operation in Sudan for the fifth year in a row. We are already overstretched because of the needs in Darfur, but we would be ready to assist and help if it is a life-threatening situation," confirmed Saleh Dabbakeh, the ICRC's spokesman in Khartoum.
But UN humanitarian chief John Holmes warned UN agencies, NGOs and the Sudanese government don't have the resources to fully replace the activities of the expelled aid groups.
swissinfo, Simon Bradley in Geneva
Experts say almost six years of fighting in Darfur has killed 200,000 people and uprooted more than 2.7 million, who now live in camps near Darfur's main towns. Khartoum claims 10,000 have died.
Some 200,000 people have also sought safety in neighbouring Chad but many of these are camped along a 600km stretch of the border and remain vulnerable to attacks from the Sudanese side.
Chad's eastern areas have a similar ethnic make-up to Darfur and the violence has spilled over the border area, with the neighbours accusing one another of supporting each other's rebel groups.
Many aid agencies are working in Darfur but they are unable to get access to vast areas because of the insecurity.
The ICC issued an arrest warrant in March 2009 for Sudan's, President Omar al-Bashir, who was indicted for war crimes and crimes against humanity.
In February 2007 the ICC indicted two Sudanese officials for war crimes relating to Darfur. Sudan says the ICC has no jurisdiction and refuses to hand over the suspects.
Sudan's rebel Justice and Equality Movement (Jem) signed a declaration of intent in February 2009 to pave the way for broader peace talks with Khartoum. But previous peace efforts have faltered, as the rebels group have splintered.
The joint African Union-UN peacekeeping mission (Unamid) took over from January 2008 from an African peace force.
The mission is currently at only 60 per cent of its mandated strength, with just 15,000 of the 26,000 planned troops and police on the ground.
Expelled international organisations
Action Contre La Faim
International Rescue Committee
Norwegian Refugee Council
Save the Children UK
Save the Children Fund US