The African Union (AU) has ceasefire monitors in Sudan's troubled Darfur region and the pan-African body has also been mediating in faltering peace talks in Nigeria between the Sudanese government and the rebels.
However, AU spokesman Assane Ba said one of his aircraft had come under attack.
"One of our helicopters has been shot," he said.
"They are firing on our helicopters. This shows that the ceasefire is not being observed. They did not comply. They have not stopped fighting."
He gave no further details and it was not clear who had shot at the helicopter, or when.
Sudan said on Sunday it would immediately and unconditionally cease hostilities in Darfur and asked the United Nations and AU to request that rebel forces do the same.
"Yes, we will inform our forces in Darfur immediately to stop any fighting, so we will not fire unless we will be attacked by the other side," Foreign Minister Mustafa Usman Ismail said in Khartoum.
Ba had accused the Sudanese government of failing to comply with a deadline to stop fighting in Darfur, describing fresh helicopter strikes on a village in South Darfur state.
The AU force commander in Darfur, Nigerian General Festus Okonkwo, told mediators government forces attacked the village of Labado on Saturday, Ba reported.
Sudan's FM Mustafa Ismail insists
the ceasefire deadline was met
"Things have changed; the latest report from General Okonkwo is that the Sudan government has not complied. He said government helicopters attacked Labado and burned the place yesterday," Ba said.
But Ismail insisted the government had met Saturday's deadline, and retaliated later after coming under rebel attack.
Okonkwo earlier said the Sudanese government had complied with the ultimatum to stop hostilities on Saturday or face the matter going to the UN Security Council.
Ba said an African country - which some mediators privately identified as Libya - was trying to negotiate a fresh initiative to persuade the fighters to withdraw and to rejuvenate the talks.
The rebels left the negotiations a week ago to protest against a renewed government offensive on their positions.
Mediators had a meeting on Sunday with Nigerian President Olusegun Obasanjo, who is also chairman of the African Union.
Obasanjo asked the mediators to give the would-be peace negotiating country more time to ask the two sides to stop fighting, Ba said.
AU chairman Olusegun Obasanjo
says more time is needed
Labado, which had been in rebel control, is about 65km east of Nyala, the capital of South Darfur state.
Thousands of Darfuris are fleeing the fighting, streaming towards Nyala town from the east, bringing reports of government bombardment with helicopters and Antonov planes.
They say government forces and militias, known as Janjawid, attacked their villages and in some cases set up bases there.
Aid community sources in Darfur say rebels have been attacking aid and goods convoys along the Nyala to al-Fasher road, where two Save the Children workers were killed recently.
Many of the villages along the road to the east were rebel strongholds before intense fighting in the past week. The offensive began after the government said rebels attacked a convoy of 500 government troops on a routine patrol in the area.
"This road is vital to the Sudanese government as it links their troops to supply routes from Khartoum," one source said, asking not to be identified.
The World Food Programme is
concerned about relief efforts
The United Nations has closed many of the roads out of Nyala to aid traffic. The World Food Programme says at least 160,000 people are cut off, which could increase if fighting spreads.
"We are waiting for the AU. We will give them a reasonable time to persuade the government. But if the attacks continue, we will be forced at some point to answer the Sudan government," said Taj al-Din Bashir Niam, from the Justice and Equality Movement (JEM) rebel group.
Niam said there were indications the government troops were advancing from Labado to other towns.