Fighting has broken out in the town of Goz Beida in eastern Chad, reports say.
The rebels want to oust President Idriss Deby's government
Up to 60 rebel vehicles were said to have entered the town, which is about 100 miles (161km) from the Sudanese border.
Rebels claimed to be in control of the town, but a government spokesman said that the rebel attack had been "repelled" by the army.
There was no information on casualties, but an aid worker said shots had been heard in the town.
"Our forces have taken over this evening the town of Goz Beida," Acheikh Ibn Oumar told the French news agency AFP.
"There was fighting but government forces didn't resist long," he said.
Mr Oumar claimed to speak for three rebel groups which had just decided to form a Union of Forces for Democracy, the agency said.
The new alliance comprises Mr Oumar's Democratic Revolutionary Council (CDR), the Union of Forces for Progress and Democracy (UFPD) and the United Front for Change (FUC).
But government spokesman Hourmadji Moussa Doumgor, while confirming that "the rebel forces infiltrated Goz Beida", said the military had fought off the attack.
In recent months, violence from neighbouring Darfur has been spilling over into Chad.
Several rebel groups hostile to Chadian President Idriss Deby also operate in the east of the country.
While this appears to be an isolated incident, observers are mindful of the last time Chadian rebels came through this region, says the BBC's Stephanie Hancock in the capital N'Djamena.
Back in April, rebels passed through eastern Chad before driving directly to the, N'Djamena and attempting to overthrow the president.
Their attack was foiled, but the rebellion is still in place and still intent on toppling Mr Deby.
Diplomatic sources also say that with the Muslim holy month of Ramadan drawing to a close, they expect security in eastern Chad to deteriorate even further.