Rebels from Chad have attacked neighbouring Central African Republic (CAR), leaving an unknown number of rebels and soldiers dead, the CAR says.
Rebel attacks have forced refugees to flee across borders in the region
Peacekeepers from a regional body were also involved in the clashes with "heavily armed" rebels in the north, the interior ministry says.
Correspondents say large parts of CAR, Chad and Sudan are devoid of authority, with many armed groups operating.
A BBC correspondent says rebels from Chad and CAR have formed an alliance.
The BBC's Joseph Benamsse in the CAR capital, Bangui, says the Chadian rebels want to be left alone in the remote northern region.
In April, the Chad rebels launched a raid on the capital, N'Djamena, before being repulsed.
"The fighting that followed was particularly intense, causing a number of deaths on both sides," the CAR interior ministry said.
A CAR military source told the AFP news agency that 71 rebels and five members of the CAR/ Cemac regional force were killed in the clashes near Tiroungoulou some 800 kilometres (500 miles) from the capital Bangui.
More than 10,000 Chadians have fled the fighting for Sudan's war-torn Darfur region in the last month, the international medical charity Medecins Sans Frontieres says.
Darfur has been a war-zone for the past three years, with pro-government Janjaweed rebels accused of genocide against black Africans.
Now the Janjaweed are attacking deep into Chad, and making alliances with some groups while fighting against others, as the area has a similar ethnic make-up to Darfur.
After April's coup attempt, Chadian police and military were withdrawn from the south, to boost the government's fragile hold on N'Djamena, leaving it almost devoid of any real authority.