Sudan is reported to have ordered two senior UN officials to leave, amid tensions between Khartoum and the peacekeeping mission in the western Darfur region.
A UN spokesperson, who spoke on condition of anoymity because he was not authorised to speak to the media, said Ali Zaatari and Yvonne Helle were ordered to leave on Thursday.
Zaatari has been UN resident coordinator at the United Nations Development Programme (UNDP) while Helle worked as the UNDP's country director.
The UNDP and Sudanese foreign ministry did not respond to requests for comment.
Stephane Dujarric, the UN spokesperson, said the UN had protested against the expulsions.
"The UN has filed a protest with the government of Sudan following their decision to request the departure of two senior UN officials from the country," he told the AFP news agency.
Dujarric said Ban Ki-moon, the UN secretary-general, "condemns" the move and urged Sudan to "immediately" reverse the decision and cooperate with UN entities in the country.
"The sanctioning of United Nations personnel sent to Sudan to carry out their duties in accordance with the United Nations Charter is unacceptable."
The head of the UN peacekeeping forces said UNAMID was unlikely to bow to Sudan's request to leave when the situation there appears to be worsening, Reuters news agency reported.
It was unclear clear why the UN officials were asked to leave, or when they would have to exit the country. The UN staffer declined to provide further details.
Zaatari, a Jordanian national, had been in Sudan for nearly two years, and Helle, who is from the Netherlands, had spent about a year heading the UNDP's office in the country.
The expulsions come as Sudan's government is locked in a dispute with the hybrid UN-African union mission in Darfur known by its acronym UNAMID.
Ties between the two have frayed over Khartoum's anger at the mission's attempts to investigate a report that government troops raped 200 women and girls in a village in Darfur on October 31.
'Exit strategy' sought
Sudan demanded UNAMID form an "exit strategy" from Darfur, where they have been deployed since 2007, and ordered it to shut a human rights office in Khartoum last month.
Zaatari and Helle's expulsion are the latest in a string of incidents with foreign aid and humanitarian workers in Sudan.
In April, the government told the American chief of the UN Population Fund in Sudan to leave for "interfering" in internal affairs.
The UN and international NGOs provide aid to some of the areas of Sudan worst affected by the conflicts wracking its peripheries.
The government is battling an insurgency in Blue Nile and South Kordofan, as well as a protracted conflict in Darfur.
Ethnic rebels rebelled against Khartoum in 2003, complaining they were being marginalised, and in the intervening 11 years the UN says some 300,000 people have been killed and two million displaced.