Rebels in northern Chad attacked government forces this week at the border with Libya, the fighters and two military sources said on Friday, although the government denied an attack had taken place.
A fledgling rebel movement, the Military Command Council for the Salvation of the Republic (CCMSR), says it seeks to overthrow President Idriss Deby, as Chad faces threats from militants and is trying to prevent the influx of extremists fleeing the Libyan conflict.
Mr Deby has been an ally of the West in the fight against Islamist militants in West Africa.
He has faced several rebellions since coming to power in 1990 at the head of an insurrection that toppled then president Hissene Habre, but there has been relative calm since 2009.
The CCMSR, which says it has 4,500 fighters, was founded in 2014 and fought its first battle against government forces earlier this month in the mining town of Kouri Bougoudi.
Its ranks include former rebels from the Darfur region of neighbouring Sudan and former political allies of ex-president Mr Habre, who is serving a life sentence in a Senegalese prison for war crimes and crimes against humanity.
The CCMSR said its fighters attacked Chadian soldiers again in Kouri Bougoudi early on Tuesday morning.
"The valiant fighters ... attacked the enemy once again in the most brutal manner," it said in a statement.
"Unfortunately, the enemy preferred to flee in total disarray, leaving behind sheep carcasses and other heaps of food."
Security Minister Ahmat Mahamat Bachir denied that the rebels had attacked the army, but two Chadian military sources told Reuters that there had been fighting. Neither the CCMSR nor the sources provided any information about casualties.
The nascent rebellion is the latest security headache for Chad which closed the border with Libya in January in the hope of barring its militants from entering.
Chad also faces threats from groups with links to Al Qaeda and ISIS, which operate across the lawless, semi-arid Sahel band, and from Nigeria-based Boko Haram militants.
Mr Deby, 66, has won election five times since coming to power, often amid accusations of fraud. Parliament approved a new constitution in April that expands his powers and could allow him to stay in office until 2033.