Chadian rebels have threatened to take up arms against President Idriss Deby's government two years after both sides agreed to stop fighting.
Chadian rebel leader Timane Erdimi said on Thursday that after two years of waiting for talks the rebels had no other options, according to the Reuters news agency.
Reuters said Erdimi made the threat by telephone from the Qatari capital Doha.
"We're tired of waiting. Our supporters on the ground are tired and are pushing us to fight given Deby's obstinate refusal. We must resume fighting," Erdimi said.
The Union of Forces of Resistance (UFR), a rebel coalition, put down their weapons after Chad and Sudan agreed to end their proxy wars in 2010 by halting support for fighters in each other's country.
The two nations agreed to work together to rebuild their border areas, a move seen aimed at bolstering security and credibility before impending elections in both nations.
The former French colony, one of the poorest nations in the world, has been rocked by humanitarian crises over the last decade including conflicts in the east and south, drought in the arid Sahel region, and flooding.
Deby seized power in a 1990 military coup and has since won a series of elections whose fairness has been questioned by international observers. He has dismissed those allegations and defended his record.
Erdimi was the leader of one of several rebel groups in a 2008 rebel coalition which attacked the Chadian capital N'Djamena in February that year, besieging Deby in his palace.
The rebels eventually withdrew, accusing former colonial power France, which has troops and planes based in Chad, of backing Deby. Paris said its forces gave intelligence, medical and logistics support to the Chadian army, but did not participate directly in combat.
Deby's critics say three polls since the coup were unfair and call him corrupt and dictatorial.
"The problems cannot be resolved unless there are negotiations between the two sides," Erdimi, who has been exiled in Doha since Sudan normalised ties with Chad, said.
"There was a deal between Chad and Sudan, but not us. We were asked to put down our weapons and begin negotiations with Deby. As soon as he saw we had sincerely put down our weapons he refused to begin talks."
Erdimi, a nephew and former aide of Deby, said that not all the rebels had disarmed and were now massing between the Sudanese-Chad border town of Tissi in the southeast to the Libyan border in the north.