The African Union has temporarily suspended all monitoring flights in southern Darfur, after one of its helicopters came under fire.
AU troops are deployed too thinly to have much impact
The helicopter was carrying a team of AU observers who were trying to verify compliance with a ceasefire on Sunday.
Thousands of civilians have been driven from their homes by a government offensive in the past two weeks.
After urgent talks with international envoys, Sudan said it would suspend operations in Darfur in west Sudan.
The helicopter sustained a number of gunshot holes. The AU says it will investigate the shooting, but at the moment it cannot say whether the fighting has stopped or not.
After announcing an end to the offensive, Foreign Minister Mustafa Osman Ismail said that troops would only respond if fired upon first.
However, he ruled out any withdrawal of government troops from positions they had recently taken from rebels.
A ceasefire deadline passed on Saturday night with no end to the clashes.
On Sunday, AU spokesman Assane Ba said it was clear that fighting was continuing in southern Darfur, although he did not say who was firing.
"One of our helicopters has been shot. They are firing on our helicopters. This shows that the ceasefire is not being
observed. They did not comply. They have not stopped fighting," Mr Ba told reporters in Nigeria's capital, Abuja.
The AU and Western ambassadors had earlier threatened to abandon peace talks in Abuja if their deadline of 1700 GMT on Saturday was not met.
Earlier on Sunday, rebel commander Colonel Omar Adam also said that attacks by Sudanese troops - backed by helicopter gunships - were still taking place in southern Darfur.
Col Adam - who is from the Justice and Equality Movement (Jem) - warned that rebels would reconsider all the agreements they had signed if Sudan's troops did not withdraw from the recently seized positions.
"Our message now is very clear. Our patience is running out... We ask the African Union to put pressure on the regime of Khartoum," Col Adam told the BBC's Focus on Africa programme.
He added said the rebels could also pull out from the Abuja peace talks.
Fighting has intensified in the last two weeks as the army pushed into previously rebel-held territory.
The peace talks are aimed at ending a conflict that has raged since February 2003 and also to force both sides to stick to a ceasefire deal signed in April.
Aid groups have been helping thousands of refugees in Darfur
The conflict in Darfur - dubbed the "world's worst humanitarian crisis" - has left about 70,000 people dead.
More than 1.5 million have fled their homes, mostly black Africans targeted by pro-government Arab militias known as the Janjaweed.
Khartoum has consistently denied arming or working alongside the militias.