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.Mustafa Allandi, a Tibu member of the ruling National Transitional Council, called for stronger government action to stop the fighting: "What is happening in Kufra now is not acceptable. The NTC should do its job and stop this."Long-standing rivalries, divided communities and plentiful weapons are plaguing Libya as the interim government struggles to impose its authority on the vast oil-producing country's ethnic groups.The Tibu tribe lives mainly in Chad but also inhabits parts of southern Libya. In February, the Zwai tribe accused the Tibu of attacking Kufra, backed by mercenaries from Chad. The Tibu said it was they who had come under attack.The remote southeast has a history of violence involving tribes. A tribal rebellion in 2009 was suppressed only after Gaddafi sent in helicopter gunships.