The fighting erupted Friday in the coastal town of Zawiya between two rival militias allied with the government of Prime Minister Abdul Hamid Dbeibah, which is based in the capital of Tripoli.

The National Oil Corp. said the fighting damaged at least 29 sites, including storage tanks, at the Zawiya refinery complex. It said an assessment was continuing to determine the extent of the damage. The refinery is a major source of domestic fuel raising fears of an energy crisis amid the heat of the summer.

The National Commission for Human Rights, a local group, condemned the clashes, which pitted the so-called Stability Support Authority against the self-styled Criminal Investigation Department in Zawiya.

It was not clear what caused the clashes.

There was no comment from Dbeibah's government on the fighting. Its rival administration voiced concern and called on armed groups to stop fighting which left unknown number of people dead.

Tensions have soared in Libya, especially in the western region, since the east-based parliament appointed Fathi Bashagha as prime minister in February. Occasional fighting between militias also occurred in the capital.

Bashagha has yet to be seated in the capital as Dbeibah remained defiant against efforts to replace his government, insisting that he will hand over power only to an elected government.

Tribal leaders and protesters in the southern region also shut down oil facilities, including Libya's largest oil field, demanding Dbeibah step down. The region is controlled by forces of east-based commander Khalifa Hifter.

The developments have raised fears that the country could return to civil war amid the ongoing standoff between the two rival governments.

The oil-rich North African country has been wrecked by conflict since the NATO-backed uprising toppled and killed longtime leader Moammar Gadhafi in 2011. The country has fragmented for years between rival administrations in the east and west, each backed by armed groups and foreign governments.