The state of emergency was announced on February 15 and included a nationwide curfew from midnight to 6am, monitoring the passage of people and vehicles, house searches and media censorship.
An alliance of three groups crossed the central African country from rear bases in Sudan's Darfur region and attacked Ndjamena to overthrow the government at the beginning of February, before being driven out by Deby's troops with indirect French military support.
A number of human rights organisations have accused Chad of arresting members of the Chadian opposition, including spokesperson Ibni Oumar Mahamat Saleh, who is still missing.
Timane Erdimi, head of the Rally of Forces for Change (RFC), told Reuters that his armed group would attack Chad's southern oil-producing Doba region unless Paris and Washington exercise pressure on Deby to talk with his foes.
"We can carry the war to the south ... if the Americans and the French don't put pressure on Deby to open an all-inclusive dialogue with political and military players," Erdimi said.
He said the Doba basin in southern Chad, where US oil company ExxonMobil heads a consortium pumping 140,000-160,000 barrels a day of oil via a pipeline to Cameroon's Atlantic coast, could become a military target unless Paris and Washington did more to achieve a political settlement.
Erdimi's RFC was part of the coalition that attacked NDjamena in February.
The coalition has since split politically, but Erdimi said they were still co-operating militarily and could "easily" strike at the oil-producing south, which has been spared attacks so far.
"The government only controls NDjamena and the town of Abeche. That's it," Erdimi said, speaking from the Sudanese capital, Khartoum.