The African Union (AU) has expressed disappointment at the failure of the two remaining rebel groups in Sudan's Darfur region to sign a peace deal.
SLA faction leader Abdel Wahid Nur refused to sign earlier in May
They ignored the latest deadline to agree to the AU-brokered deal signed last month by the main faction of the rebel Sudan Liberation Army.
The AU says continuing violence is the work of those opposed to the deal.
A BBC correspondent says the last month shows that any deal without both SLA factions means little on the ground.
The AU says it will consider sanctions for those responsible for renewed violence.
AU head Alpha Oumar Konare said it was with "deep regret" that he learnt that the SLA faction loyal to Abdel Wahid Mohammed al Nur and another smaller group, the Justice and Equality Movement, had ignored Wednesday's midnight deadline.
The rebel leaders are calling for more negotiations.
"We are calling on the United Nations and international mediators to be patient, not to hurry up, not to force an unacceptable peace on people of Darfur," Jem leader Ibrahim Mohamad Khalil said, Reuters news agency reports.
But Mr Konare said the AU's peace and security council will now meet to decide whether to impose sanctions on the rebel leaders.
The BBC's Jonah Fisher in Khartoum says with 7,000 troops in Darfur, the AU has seen at first hand what Mr Konare called the progressive deterioration of the security situation especially in the last few weeks.
Disturbances in camps had been instigated by opponents of the deal, with AU personnel targeted and on two occasions killed, Mr Konare said.
He said that future violations of the ceasefire agreement would not be tolerated and that AU forces should operate proactively to defend themselves and civilians.
Western donors would like to see the small, poorly equipped AU force replaced but moves to bring in a bigger, tougher United Nations mission have so far been opposed by the Sudanese government.
The two smaller rebel factions have demanded changes to the text of the agreement aimed at ending the conflict, in which at least 200,000 people have died.
Three years of fighting has forced some two million people from their homes, in what aid agencies say is one of the world's worst humanitarian crises.
The two rebel groups say the current deal is unjust and are asking for:
- More political posts in a proposed transitional government
- A say in the disarmament of pro-government militias
- Compensation for victims of the conflict.
The rebels took up arms in February 2003, accusing the government of discriminating against Darfur's black Africans in favour of Arabs.
The Sudanese army and the pro-government Janjaweed militia then began attacking civilians in Darfur, driving people from their homes and attacking refugee camps.
A lack of money and insecurity means aid workers cannot reach parts of the region.
Sudan denies arming the Arab militias and says the problems have been exaggerated.