Wissam Kilani, a 22-year-old Libyan fighter, left the front line on Wednesday afternoon just long enough to restock with two rocket-propelled grenades, before heading back into the latest of the ethnic and tribal conflicts convulsing the new Libya.

The government in Tripoli said it would intervene with force if the two sides do not halt hostilities, but it is looking enfeebled against the powerful mix of divided communities, old vendettas and plentiful weapons that is driving this conflict.

I will keep on fighting, said Kilani, one of the fighters from the town of Zuwara in western Libya who for four days have been battling their neighbours from the nearby settlements of Al-Jumail and Regdalin.

We don't want to make peace with them, he said, his leg bandaged where he was wounded two days ago. This is our land and we will defend it.

Four days of fighting between the rival communities around Zuwara have killed 18 people on both sides and wounded hundreds, a government spokesman said on Wednesday.

The violence has exposed how volatile Libya remains, six months after a revolt last year ended Muammar Gaddafi's rule, and how the National Transitional Council (NTC), the interim leadership, is struggling to impose its authority.

In Tripoli, the army chief of staff said a government intervention force was in the area of Zuwara.

These forces are ready to stabilise the area, Youssef al-Mangoush told a news conference. The government has mandated the chief of the army to use force if needed to provide security.

But the only visible signs of a government security presence was an air force plane flying over the town, and a small contingent of troops well back from the front line. Mangoush said they had retreated a little because of the fighting.

In the meantime, the violence carried on with no sign of the ceasefire that the NTC had been trying to broker.

A Reuters team that entered Zuwara said they could hear the sound of Russian-made Grad rockets, as well as the crack of rifle fire. Heavier weapons were being used too, including 106 mm anti-tank guns and anti-aircraft guns that have been adapted to fire at targets on the ground.

Between the palm trees and lush green vegetation around Zuwara, a town on the Mediterranean coast about 120 km (75 miles) west of the Libyan capital Tripoli, plumes of smoke rose up from where munitions struck the earth.

The U.N. mission in Libya said it was deeply concerned about the violence around Zuwara and called on all sides to cease hostilities immediately.

ROOTS OF THE VIOLENCE

The violence has its roots in last year's rebellion against Gaddafi, which in Zuwara, as in many other parts of the country, set one neighbour against another.

Zuwara's population, made up largely of members of the Berber ethnic group, opposed Gaddafi during the revolt. Their neighbours to the south are mainly Arabs who had been loyal to Gaddafi.

That created mistrust and resentment which ignited at the weekend when, according to one account, a group of men from Zuwara out hunting for game shot dead a man from Al-Jumail by mistake. The hunters were briefly detained and, say people in Zuwara, mistreated, setting off the violence.

At a hospital in Zuwara, a nurse said two people from the town had been killed in the fighting on Wednesday.

Outside the hospital, ambulances and pickup trucks, some mounted with anti-aircraft guns, were bringing in the wounded from the front line.

One wounded fighter held up his bloodied hand to loud cheers of Allahu Akbar! (God is greatest) from a small crowd of people outside. Some young fighters cried, while women scrubbed blood and dirt off stretchers on the ground so they could be re-used.

Rahil Nasser, a 27-year-old woman, went to the hospital because her cousin had been killed in the fighting. She urged the NTC to intervene to protect the town.

We have to activate the army to protect people, she said, tears streaming down her face. Our children, our revolutionaries are dying ... We are going to fight to defend ourselves.

The NTC's ability to intervene though is limited because its own modest security forces are out-gunned by militias who answer only to their own commanders.

The security force sent by the NTC - a mix of men with national army insignia on their uniforms and militia men from the city of Misrata - had arrived but was parked up on the western edge of Zuwara, well away from the fighting.

Reuters reporters were unable to cross the front line to speak to people on the opposing side, in Regdalin and Al-Jumail, and get their version of events.

Fighters from Zuwara say their opponents are sympathisers of Gaddafi, who was killed in October soon after he was captured in a storm drain near his home town of Sirte.

Near the front line on Wednesday, two fighters produced a green flag and a banner with the words Long live Al Fatah, which they said they had found in Regdalin.

Al Fatah was the name Gaddafi gave to the 1969 coup which brought him to power, while the green flag was Libya's national symbol under his rule.

However, there was no hard evidence that the fight was over support for Gaddafi. Often when local conflicts have flared up in Libya since the revolt, one side describes their opponents as Gaddafi loyalists to discredit them.

In another confrontation that has underlined Libya's fragility, about 150 people were killed in clashes over the past week between rival tribes in the southern city of Sabha.

(Additional reporting by Taha Zargoun; Writing by Christian Lowe; Editing by)