Fighting has erupted in the Darfuri town of Muhajiriya, controlled by the one rebel faction to sign a peace deal with the government in May last year.
A number of rebel movements have threatened not to attend the talks
The Sudan Liberation Army faction run by Minni Minnawi came under attack from government troops.
The SLA say dozens have died, and others are being treated for wounds by the African Union peacekeeping force.
Amnesty International says Sudan's Armed Forces are gathering in large numbers in at least six Darfuri towns.
The London-based human rights organisation said it had received credible reports that the Sudanese army was close to the towns of Kornoy, Um Baru, Kutum and Tine in northern Darfur.
A separate report spoke of a clash between soldiers and a rebel group in Tine, which is close to the Chadian border.
The SLA says the fighting comes as "a stab in the back" for their alliance.
Observers say the upsurge in fighting is an attempt to gain ground ahead of the peace talks due to take place in Libya on the 27 October.
The commander of the 7,000-strong African Union force, General Martin Luther Agwai, told the BBC in the Sudanese capital, Khartoum, that at least 24 people were injured in the attack on Muhajiriya.
He initially said he believed Sudanese government planes had bombed the town on Monday but, later on, the AU suggested there may not have been any aerial bombing after all.
Minni Minnawi was the only rebel leader to back the failed Darfur peace agreement in Abuja last year. He is now part of the government as special assistant to the president.
The attack on the SLA-controlled town comes just 10 days after the African Union base in nearby Haskanita was raided by armed men, presumed at the time to be rebels.
Ten AU peacekeepers were killed in the raid, while equipment was destroyed or looted.
The BBC's Amber Henshaw in Khartoum says this latest spate of violence does not bode well for the talks.
On Monday the UK warned rebel groups in Darfur they could be excluded from the peace process if they boycotted the Libyan talks.
One key rebel faction says it will not enter peace talks until the promised UN peacekeeping force is deployed.
The Paris-based Abdul Wahid al-Nur faction of the Sudan Liberation Movement/Army told the BBC that Darfur needs the international community to live up to its responsibilities to stop the killing.
The Justice and Equality Movement, one of Darfur's main rebel factions, also said the British threats were not helpful.
UN officials have appealed for helicopters and trucks for the new UN-African Union mission in Darfur to replace the poorly equipped 7,000 AU observers currently deployed.
General Agwai called on the different parties to lay down their arms to try to find a peace agreement.
At least 200,000 people have died in Darfur during a four-year conflict and more than two million have been forced from their homes.