Military observers will again try to investigate reports that Sudan's air force has bombed a town in the Darfur region, in violation of the ceasefire.
AU monitors deny turning their backs on Darfur's civilians
Sudanese soldiers turned away African Union monitors from the town of Shangil Tobaya on Thursday, saying it was not safe, an AU official told the BBC.
About 100 people, mostly women and children, are believed to have been killed in the air raid.
Some 70,000 people have been killed in the two-year conflict.
A further two million people have fled their homes, mostly black Africans, who accuse pro-government Arab militias of committing atrocities.
The Sudanese government denies backing the Janjaweed militias, and blames rebels for starting the conflict.
Human rights group Amnesty International has urged the Sudan government to either charge or release four men it says have been held in a hole in the ground in South Darfur for the past six weeks.
It says they have been flogged and beaten with sticks and gun-butts by soldiers and militiamen. A fifth man has died from his injuries, Amnesty says.
The air raid, which some fear signals an upsurge in fighting, has been condemned by the UK.
British Foreign Secretary Jack Straw said it was a "reprehensible" attack.
"The majority of the victims were women and children. We are extremely concerned," a spokeswoman for the UN envoy to Sudan, Jan Pronk, told the BBC.
The rebels do not have an air force and AU officials have hinted that the government was responsible.
The government has accused rebels of launching attacks in the area around Shangil Tobaya, on the border between North and South Darfur.
A United States official said all sides were involved in the latest fighting.
"All the parties, the government of Sudan, the militias that are allied with the government and the rebels, are to blame for this increase in violence," said state department spokesman Richard Boucher.
The head of the AU mission in Sudan, Baba Gana Kingibe, denied that the monitors were ineffective if they could only operate in areas which the government said was safe.
"The AU troops are monitoring compliance with a ceasefire, they are not a peace enforcement operation," he told the BBC's World Today programme.
"We wouldn't want to put our troops in harm's way."
AU monitors are also investigating reports that 100 more people were killed in a raid by Janjaweed militias in the nearby village of Hamada last week.
Some 9,000 people have fled the area, a UN official said.
The AU has some 1,400 troops and military observers in Darfur, and Mr Kingibe said he hoped they would be increased to 3,200 next month.
A UN investigation on whether genocide has been committed in Darfur has been completed and is expected to be made public next week.
This may include a sealed list of those accused of war crimes.
The latest fighting has produced more refugees, the UN says
The US has said that genocide is being committed and has again started to lobby for a UN resolution threatening sanctions against Sudan.
Previous attempts to threaten sanctions have been blocked by China, which has oil interests in Sudan, and Russia, which has sold arms to the government, according to lobby group Human Rights Watch.
Two rebels group took up arms in 2003, accusing the Arab government of neglecting Darfur.
The rebels have since split into four factions and peace talks with the government have made little progress.