The peacekeeping force in Darfur is understrength and under pressure
United Nations chief Ban Ki-moon has urged Sudan to co-operate with the UN and ensure the security of peacekeepers and humanitarian workers in Darfur.
His call came a day after a prosecutor for the International Criminal Court (ICC) accused Sudanese President Omar al-Bashir of genocide.
He denies the charge, and Sudan says it will seek to block the work of the ICC.
Meanwhile, Mr Ban said in a new report he was "deeply disappointed" by a lack of progress on ending strife in Darfur.
The report, released on Tuesday but dated 7 July, did not mention the request by prosecutor Luis Moreno-Ocampo for the ICC to issue an arrest warrant for the Sudanese president.
Mr Bashir, who says the accusations are lies, is accused of genocide, crimes against humanity and war crimes in Darfur.
The UN is withdrawing about 200 non-essential staff from the joint UN-African Union Darfur mission, Unamid, describing it as a precautionary move after the genocide accusation and recent violence.
'Should be protected'
Speaking in Berlin after talks with German Chancellor Angela Merkel, Mr Ban urged Sudan to "fully co-operate" with the UN.
The UN secretary-general said Mr Bashir must ensure "that the United Nations peacekeeping operations should be able to carry [out] their duties and missions as mandated by the Security Council".
He went on: "At the same time, there are 16,000 humanitarian workers who are supporting more than 4 million refugees and internally displaced people.
"This is a huge operation, their efforts should also be protected."
Some 300,000 people have died as a result of the conflict in Darfur since 2003, while more than two million people have fled their homes, the UN estimates.
Sudan's government denies mobilising Arab Janjaweed militias to attack black African civilians in Darfur since rebels took up arms in 2003.
In the report released on Tuesday, Mr Ban said he was very concerned about reports that a Sudanese rebel group appeared to be using child soldiers.
Protesters turned out in Khartoum in support of President Bashir
The Sudanese government had reported that after an attack on Khartoum in May by the rebel Justice and Equality Movement (Jem), some of those it captured were child soldiers.
Mr Ban said the tensions between Sudan and its neighbour Chad, which Khartoum accuses of backing Jem, "should also be highlighted as a source of considerable instability in Darfur and volatility on both sides of the border region".
Earlier, President Omar al-Bashir's most senior adviser told the BBC the ICC allegations were designed to generate hostility between tribal groups in Darfur.
Ghazi Salaheddin warned that if the ICC pursued the case, it could jeopardise relations between Sudan and the UN.
Thousands of Sudanese took to the streets of Khartoum on Tuesday to rally in support of their president.
Judges at the ICC, an independent body, are yet to decide if there are reasonable grounds to issue an arrest warrant against Mr Bashir.
In its first reaction, China expressed grave concern over the ICC prosecutor's decision to seek the arrest of Omar al-Bashir, saying it could undermine efforts to bring stability to Sudan.
ACCUSATIONS AGAINST BASHIR
Killing members of the Fur, Masalit and Zaghawa ethnic groups
Causing these groups serious bodily or mental harm
Inflicting conditions of life calculated to bring about these groups' physical destruction
Crimes against humanity:
Attacks on civilians in Darfur
Pillaging towns and villages
Meanwhile Russia called for "restraint" from all sides.
The Peace and Security Commissioner for the African Union, Ramtane Lamamra, has flown to Sudan for a meeting with Mr Bashir and other members of the government.
The AU Commission expressed concern that "hard-won gains made in the search for peace and reconciliation in the Sudan" could be jeopardised.
Foreign ministers of the 15 countries currently serving on the AU's Peace and Security Council are expected to meet in Addis Ababa, the Ethiopian capital where the AU is based, next week.
The charges against President Bashir put African countries in an acutely difficult position, says the BBC's Liz Blunt in Addis Ababa.
They supply almost all the troops for the joint AU/UN peacekeeping force in Darfur, and are also the countries most likely to be called upon to carry out any arrest warrant, she says.