The Libyan National Army (LNA) has launched a military operation to secure the southern border, it said on Friday, after fighting near the area resumed between the government of Chad and a rebel group trying to unseat it.

Chad's President Mahamat Idriss Deby said on Sunday that the army was again fighting the Libya-based Chadian Front for Change and Concord (FACT) group, which quit a ceasefire last week amid clashes.

FACT had fought alongside the LNA as one of many mercenary groups involved in Libya's civil war, researchers on Libya say, but they were on opposing sides during fighting two years ago when Deby's father was killed.

LNA spokesperson Ahmad Mismari said the operation would involve land and air forces. An LNA media unit distributed photographs of Haftar's son, Saddam Haftar, overseeing the operation with other LNA officers.

The media unit said the LNA had expelled members of the Chadian opposition and their families from a residential area they were using in a desert town 300km (200 miles) north of the border with Chad.