TRIPOLI - Thirteen people were killed in a second day of clashes between Libyan soldiers and tribesmen in the remote southeast, a security official said, highlighting the challenges the new government still faces in imposing its authority.

Violence erupted in the early hours of Saturday in the city of  Al Kufra, near Libya's borders with Chad and Sudan, where armed forces were sent in February to quell fighting in a long-standing rivalry between the Tibu and Zwai tribes.

Bouts of violence in the southern Sahara and in the mountainous west have shown how volatile Libya remains following last year's demise of Muammar Gaddafi, who had long played off one tribe or clan against the other to weaken their power.

A Kufra official on Saturday said the Tibu had launched an attack on the city, around 1,100 km (680 miles) southeast of  the capital Tripoli. A Tibu representative said it was the tribe that had come under attack.

"The Tibu tried to attack the city at 4 a.m. (0200 GMT) this morning with tanks and weapons. Three soldiers and six citizens were killed.

There are many injured," Abdelbari  Idriss, a security official in the city, said by phone. "The clashes have continued since this morning."

He said four were killed on the Tibu side, which was also confirmed by a Tibu representative. "The fighting is mainly  around the Gadervi area. Shooting can be heard,"  Mohammed Rezzy, a Tibu representative, said referring to a Tibu area.