Sudan's army has ignored a deadline to end fighting in the war-torn Darfur region, the African Union (AU) says.
AU troops have failed so far to prevent the fighting
Spokesman Assane Ba told the BBC that the army was engaged in a fresh battle with rebels in southern Darfur.
The AU and Western ambassadors had threatened to abandon peace talks in Nigeria's capital if their deadline - that passed at 1700 GMT - was not met.
In response, Sudan said its operations were aimed at clearing the roads to allow access for humanitarian aid.
A statement by the Sudanese government delegation in Abuja added that Khartoum had now halted its military operations and had begun withdrawing troops to their former positions.
More refugees arrive at camps in Darfur every few minutes
However, the claim had yet to be verified, the BBC's Anna Borzello in Nigeria reports.
Fighting has intensified in the last two weeks as the army pushed into previously rebel-held territory.
On Tuesday, rebels walked out of the talks in protest at a Sudan government offensive.
The AU joint commission overseeing the peace process in Sudan on Friday demanded that both sides immediately end fighting which it said violated a ceasefire agreement signed in April.
Speaking from Abuja, Mr Ba told the BBC's Newshour programme that AU ceasefire monitors in Darfur had informed international mediators that "a lot of " government troops were still on the offensive late on Saturday.
"Helicopters are firing around Labado, while looting is still going on," Mr Ba said.
The spokesman said mediators in Abuja would discuss how to react to the reports of fighting on Sunday, warning that any violation of the ceasefire would also be reported to the United Nations Security Council.
Earlier on Saturday, Sudanese Foreign Minister Mustafa Osman Ismail said "the government military operations against the rebels will stop only if the rebels pull out of the areas they occupied" after the truce was agreed.
He also said the rebels must "refrain from launching attacks on civilians and public property".
Thousands more have been displaced from their homes adding more people to already overcrowded camps, says the BBC's
Jonah Fisher in Nyala, the South Darfur state capital.
He adds that people fleeing their villages have spoken of attacks by government forces and pro-government militias, some involving helicopters and Antonov bombers
'Worst humanitarian crisis'
The peace talks are aimed at ending a conflict that has raged since February 2003, when rebels began attacking government targets, claiming that the region was being neglected by Khartoum.
About 70,000 people have since died in Darfur, dubbed the "world's worst humanitarian crisis".
More than 1.5 million have fled their homes, mostly black Africans being targeted by pro-government Arab militias known as the Janjaweed.
Khartoum has consistently denied arming or working alongside the militias.