Rebel forces in Chad say they have captured the eastern town of Biltine, hours after another group said it had taken control of nearby Abeche.
Chad's army has been unable to stop attacks in the east
The government has denied that either town is under rebel control, although the army said it had withdrawn troops from Abeche for tactical reasons.
Several rebel groups in eastern Chad want to depose President Idriss Deby.
Observers say this appears to be the first time two rebel groups have launched co-ordinated attacks.
The BBC's Stephanie Hancock in the Chadian capital, N'Djamena, says this could prove a turning point in Chad's rebellion.
A spokesman for UN Secretary General Kofi Annan said he was concerned by the reports of violence in Chad.
"The secretary general reiterates his condemnation of any attempt to seize power by force. He urges the Chadian protagonists to engage in dialogue," the spokesman said.
The first attack came at around dawn in Abeche, about 700km (440 miles) east of N'Djamena.
Aid workers and residents reported heavy gunfire as rebels approached the city, and small arms fire was heard in Abeche itself.
The rebel group involved, the Union of Forces for Democracy and Development (UFDD), later issued a statement saying its men had taken over Abeche with little resistance from the Chadian army.
However, the government continues to claim that the rebels are not in control of what is a key town in the east.
Speaking on national radio, the Chadian army's second-in-command said troops had withdrawn to the outskirts of the town for tactical reasons and to avoid civilian losses.
He urged people to stay calm and accused neighbouring Sudan of backing rebel forces in Chad.
The UFDD's leader, Mahamat Nouri, told the BBC he would continue to fight until his men had captured Chad's capital, N'Djamena.
Hours later, a second rebel alliance claimed that it had taken control of the town of Biltine, some 80km (50 miles) to the north of Abeche.
Chad's defence minister told the BBC a column of rebel vehicles had been detected in the Biltine area but denied rebel claims the town had been taken.
Rebel groups want to oust President Idriss Deby's government
On Friday there were reports of as many as 200 rebel vehicles circulating in eastern Chad and the Chadian army was on full alert.
The last major rebel attack was in April, when rebels drove from eastern Chad to N'Djamena before being overpowered.
Much of Chad is under a state of emergency, after ethnic clashes which are reported to have killed hundreds of people in recent weeks.
The French embassy in Chad issued warnings to its citizens, following reports that rebels in the east were planning a new offensive and moving west.
Conflict in Sudan's Darfur region has spilled across the border into Chad, which accuses the country of exporting its violence. Khartoum denies this.
The UN estimates that more than 200,000 refugees from Sudan are in Chad and that more than 50,000 Chadians have been displaced by fighting between Arab and non-Arab groups.
Sudan denies claims that it backs Chad's rebels, while Chad denies supporting the black African rebels in Darfur.