Rebels in northern Chad attacked government forces at the border with Libya, fighters and two military sources said, although the government denied an attack happened.
Fledgling rebel movement, the Military Command Council for the Salvation of the Republic (CCMSR), seeks to overthrow President Idriss Deby as Chad faces threats from jihadists and is trying to prevent the influx of militants fleeing the Libyan conflict.
Deby is an ally of the West in the fight against Islamist militants in West Africa.
He 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 Kouri Bougoudi.
Its ranks include former rebels from the Darfur region of neighbouring Sudan and former political allies of ex-president Habre, serving a life sentence in a Senegalese prison for war crimes and crimes against humanity.
CCMSR fighters attacked Chadian soldiers again in Kouri Bougoudi on Tuesday.
“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 the rebels attacked the army, but Chadian military sources told Reuters there was 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 militants from entering.
Chad also faces threats from groups with links to al Qaeda and Islamic State, which operate across the lawless, semi-arid Sahel band and from Nigeria-based Boko Haram militants.
Deby (66) won elections five times since coming to power often amid accusations of fraud. Parliament approved a new constitution in April expanding his powers and could allow him to stay in office until 2033.