Peacekeepers have refused to leave a rebel-held town in Darfur, despite warnings from the Sudanese government that an attack is imminent.
About 5,000 civilians took shelter at the UN-African Union base after the government said the army was preparing to take Muhajiriya from rebels.
A spokesperson for the peacekeeping force - called Unamid - said they would not leave civilians unprotected.
Mediators are talking to both sides to try to prevent more fighting.
The rebel Justice and Equality Movement (Jem) seized Muhajiriya two weeks ago, sparking fierce fighting, including air strikes.
The six-year conflict has left at least 2 million people homeless.
'Endangering civilians'
"We are not going to leave while there are thousands of displaced people around our camp," Unamid spokesman Noureddine Mezni told Reuters news agency.
"The Sudanese government should be aware that their actions are endangering civilians and Unamid."
On Sunday, Khartoum had warned peacekeepers to leave the town.
"We are not ordering them around, we are asking them," said Akuei Bona Malwal, Sudan's ambassador to the African Union.
"It's sort of like informing them: 'Something will be happening here,'" the AP news agency reports him as saying.
Meanwhile, Tahir al-Feki, of the Jem rebels, said they were expecting "a large attack."
"They [government forces] are bringing tanks so they must be preparing to pound the town," he told Reuters.
Escalation
The Unamid force remains at only about half of its planned 26,000 strength, a year after the UN took joint control of the mission.
The recent fighting around Muhajiriya has marked an escalation in the ongoing Darfur conflict, almost six years after it began.
It comes as the International Criminal Court is considering whether to issue an arrest warrant for Sudan's President Omar al-Bashir on charges of orchestrating war crimes in Darfur.
The UN says up to 300,000 people in Darfur have died and more than 2.2 million have been displaced since the uprising against Sudan's Arab-dominated government started in February 2003.
Khartoum says just 10,000 have died. It says the scale of the suffering has been exaggerated for political reasons.
