The authorities in the western Sudanese state of Darfur have imposed a curfew and banned public gatherings after peace talks collapsed last week.
The aid agency pull-out will hit thousands of refugees
The BBC's Alfred Taban in Sudan says they are preparing for the worst and aid agencies are also pulling out.
Over 600,000 people have fled the fighting and are in dire need of food, shelter and medicines.
The insecurity in Darfur comes as the government and southern rebels are said to be close to ending their long war.
Diplomats have described the fighting in Darfur as "ethnic cleansing" with Arab militias, possibly backed by the government, destroying entire villages inhabited by dark-skinned people who speak African languages.
The governor of western Darfur state, Major General Suleiman Abdullah Adam, has issued a decree imposing a curfew on Geneina, the main town in the state.
Nobody is allowed to move inside the town from 2100 local time (1900 GMT) to 0700 and vehicles are prohibited from entering the town from 1900 until 0900.
The government also banned all gatherings not cleared by the authorities.
According to Governor Adam, these measures are necessary to safeguard security following the collapse of talks between Darfur-based rebels and the government aimed at extending the ceasefire.
Three days ago the governors of Northern and Southern Darfur states imposed similar restrictions for movement in the main towns and declared a general mobilisation to confront any attack following the failure of the Geneina talks.
United Nations agencies have pulled out their staff from Western Darfur state following disagreement over how to respond to the security situation there.
A UN official said the army wanted to escort the food convoys moving through the state, something which is unacceptable to the UN.
Last month rebels seized several relief workers belonging to the Swiss-based aid agency Medair.
They were released after several days. The withdrawal of UN personnel and other relief officials from Western Darfur state is likely to worsen the already disastrous humanitarian situation in the area.