At least 21 people died and 50 were wounded in Libya in four days of clashes between rival factions in the southern city of Sabha, a health official said on Sunday.

According to residents and local reports, the latest bout of violence erupted between two tribes after an incident in which a monkey that belonged to a shopkeeper from the toppled dictator Moamer Gadhafi's Gaddadfa tribe attacked a group of schoolgirls who were passing by.

The monkey pulled off one of the girls' head scarf, leading men from the Awlad Suleiman tribe to retaliate by killing three people from the Gaddadfa tribe as well as the monkey, according to a resident who spoke to Reuters.

City officials could not be reached to confirm the accounts.

Nasser al-Jehimi of the medical center in Sabha told AFP that 21 people were killed and around 100 others injured in clashes since Tuesday.

"There was an escalation on the second and third days with the use of tanks, mortars and other heavy weapons," a resident told Reuters by telephone, speaking on condition of anonymity because of the denigrating security situation.

"There are still sporadic clashes and life is completely shut down in the areas where there has been fighting."

Like other parts of Libya, Sabha has been periodically plagued by conflict since the uprising that toppled Muammar Gadhafi five years ago splintered the country into warring factions.

In the Sabha region, a hub for migrant and arms smuggling in Libya's often neglected south, militia abuses and the deterioration of living conditions have been especially acute.

The Gaddadfa and the Awlad Suleiman represent the most powerful armed factions in the region.

Salah Badr, a member of Sabha city council, declined to give specifics but noted that relations between the two tribes had been strained for many years.

"A minor incident sparked the fire," he told AFP.

During the latest clashes, which took place in the city center, initial attempts by tribal leaders to calm the fighting and arrange a ceasefire so that bodies could be recovered had failed, residents said.

"There are women and children among the wounded and some foreigners from sub-Saharan African countries among those killed due to indiscriminate shelling," a spokesman for the Sabha Medical Center said.

The city lies about 660km (410 miles) south of Tripoli. Regular inter-tribal clashes in Sebha have left several dozen people dead.