Mohammed Tirgani, a spokesman, was asked on Wednesday whether his Justice and Equality Movement (JEM) would agree to the May 5 deal which only one rebel movement in the troubled region of western Sudan has so far signed.
"No. Our demand is not met and if it is not changed then we will not sign," he told AFP by telephone.
He would not elaborate on what changes he wanted, but the two factions have said they want more political posts, better compensation for the victims of the conflict and a say in disarming the government-armed Arab militia, who are responsible for much of the violence on the ground.
Asked whether that meant the JEM would continue fighting, Tirgani said: "We will see how things get on ... we are not worried." He was referring to African Union sanctions due to be imposed if his group does not sign by midnight on Thursday.
Talks intensified on Wednesday to convince the two factions holding out against the Darfur peace deal.
Only one faction of the main Darfur rebel group, the Sudan Liberation Movement (SLM), signed the peace deal negotiated in Nigeria this month, leaving out a dissenting SLM faction and the Islamist JEM.
Noureddine Mezni, an African Union spokesman in Khartoum, said: "The day will end at midnight so we still have time and we still wish to see others joining the peace process."
Abdel Wahed Mohammed al-Nur, the other rejectionist faction leader, is in the Kenyan capital, Nairobi.
While Minnawi's faction has the most firepower in Darfur, al-Nur is from the region's largest Fur tribe, and analysts fear he may cause a split on ethnic lines if he does not sign up.
Only one Darfur group has so far
signed the May 5 Abuja accord
Mezni said the AU Peace and Security Council would decide what action, if any, to take against those who did not sign.
The council will meet in the coming days although no date has been set.
Meanwhile, the Sudanese Organisation Against Torture said police had opened fire on Darfuris in the Otash camp in South Darfur on Monday, killing one and wounding three.
In nearby Kalma, police beat and arrested dozens of demonstrators.
Three years of civil conflict in Darfur have left about 300,000 people dead and 2.4 million homeless.