(Re-leads with clashes)
TRIPOLI, March 18 (Reuters) - Armed clashes broke out at an oil field belonging to Libya's Waha Oil on Monday when a militia attacked the security forces guarding it, a company source said.
It was the second such attack in weeks as OPEC member Libya struggles to secure its vital energy industry.
"The clashes are at the Dahra field. (The guards) came under attack. It has been going on since this evening," the source said. "We are now taking steps to see how to move workers from the field for their security."
The source said a militia had fired at men working for the oil protection force guarding the field. Many of the men there are believed to be from the eastern town of Ajdabiyah.
The source said he believed the feud was over who would guard the facility. He also had received reports that an exchange of fire could also be heard at the nearby al-Ghani field, which belongs to another Libyan oil company, Zueitina.
Zueitina officials could not immediately be reached for comment.
It was not immediately clear where the militia was from but the source said it was from around the coastal city of Sirte.
The fighting comes just weeks after armed clashes at the northwestern Mellitah gas complex, which briefly halted Libyan gas exports to Italy for several days.
Libya has set up a special force, the Petroleum Facilities Guard, to secure its energy installations. The 15,000-strong force is mainly made up of former rebel fighters from the 2011 war that ousted Muammar Gaddafi.
But security remains precarious in a country awash with weapons and full of armed groups who refuse to lay down their arms.
Earlier on Monday, Deputy Oil Minister Omar Shakmak said a protest at another field belonging to Waha, Gialo 59, had not disrupted output as demonstrators demanding local hiring of vehicles blocked its entrance for an eigth day.
"So far it has not affected operations... (Oil production) has not been reduced. The operation is still as it is," Shakmak told Reuters.
Libya's state news agency LANA quoted a statement by Oil Minister Abdelbari al-Arusi saying output had not been affected at Gialo 59. An oil industry source last week said drilling and production had been affected there.
The protesters, estimated at around 100 from the nearby town of Jalu, have been calling for Waha to use locally hired vehicles and drivers at the field. They have blocked the entrance since March 11, stopping trucks there from entering.
The Waha company source said the protest was continuing on Monday and five drilling sites had now been affected by the demonstration. The field has a capacity of 120,000 barrels per day (bpd) and 70 million cubic feet of gas per day.
"If the protest continues, drilling will be affected at two more sites," the Waha source said. "Equipment, food, fuel cannot be transported because of this."
Waha's total production capacity is more than 350,000 bpd. The protest and clashes are the latest disruption in the oil industry which provides the lion's share of Libya's income.
In a series of incidents in recent months, activists and local militia have disrupted operations of Libya's energy sector calling for better living conditions or more regional autonomy.
The disruptions have affected the OPEC member's ability to return to pre-war production levels of 1.6 million bpd, although output has climbed back faster than expected. In July protesters forced the closure of three major oil terminals.
In a separate protest, Arusi said late on Sunday truck drivers who transport fuel to Benghazi's Benina airport were on a walkout, forcing the authorities to bring the product from Tripoli at a higher cost to ensure operations. (Reporting by Ali Shuaib and Marie-Louise Gumuchian; Editing by Michael Roddy)