SABHA, Libya (Reuters) - Sitting on cement blocks, surrounded by shisha pipes and machine guns, a dozen or so tribesmen guard a makeshift checkpoint outside the main city in Libya's desert south.
They are there to guard against smugglers and criminals, who have multiplied since Muammar Gaddafi's downfall in the 2011 war. They also say they are ready to battle Islamist militants that Libya's neighbors and Western nations fear are crossing the North African country's porous borders.
"If I hear al Qaeda is here, I will kill them. We know what happened in Mali and we won't allow it here, even if we only have rifles," Mohammed Wardi, 25, said as a war movie blasted from an old television nearby. "We are here to protect Libya."
A French-led military campaign this year broke Islamists' hold over the northern two-thirds of Mali, killing hundreds of al Qaeda-linked fighters and pushing others into neighboring states like Niger and eventually Libya, security officials say.
The men with Wardi are from the Tibu tribe, a black African ethnic group that also lives in Chad and Niger, which along with ill-trained tribal militias of former rebel fighters and a poorly-equipped national army are trying to maintain security in Libya's southern desert hinterlands.
The long-neglected region, with borders stretching more than 2,000 kms and home to major oil fields, has grown more lawless as the country's new rulers - hundreds of miles away in Tripoli - struggle to impose order on a country awash with weapons.
The south has seen rising violence, weapons and drug trafficking and an influx of illegal immigrants, leading the national assembly to declare the region a military zone, a decree the weak government has little power to enforce.
"The south is dying and the government is ignoring us. Crime is rampant, there are tribal animosities, smuggling and we are worried that what is happening in Mali will spread here," said a local government official, who declined to be identified.
"We are free of Gaddafi but we are prisoners to chaos."
Even under Gaddafi, the south was poorly patrolled and smugglers have long used the area - a crossroads of routes to Chad, Niger and Algeria - for trafficking drugs, contraband cigarettes and people to Europe.
But now the traffickers, who also specialize in weapons, fuel, stolen vehicles and subsidized food, are as well-armed as the security forces tasked with catching them.
"We have patrol planes, convoys of cars but the area is very big," said a senior army source at the base for the south's military governor. "Sometimes phones don't work well and we need better equipment - planes, cars, weapons even binoculars."
Adding to the lack of equipment, the militias the state relies on - especially in the harsh desert terrain its soldiers do not know - are rife with long-standing grievances.
During his 42-year iron-fisted rule, Gaddafi often played off one tribe or clan against the other and tensions persist. Last year fighting between Tibu, oasis farmers by tradition, and Arab militias in Sabha and Kufra killed more than 150 people.
Skirmishes still erupt over control of smuggling routes, sometimes by the groups supposed to be catching the culprits.
In towns such as Sabha and Obari, a remote outpost 200 kms away, police struggle to rein in crime, compounded by unemployment, drug abuse and plentiful weapons.
Military convoys and bases have been attacked. Last month, Sabha airport was briefly shutdown by angry Tibu protesting against the disappearance of a militia leader.
The main prison for the southwest is in Sabha but it holds just 95 criminals. It has been attacked with rocket-propelled grenades and prisoners broke free earlier this year.
"Most of the prisoners came back as they were too afraid to be out on the streets," Mohammed Ali Azbari, who manages the former rebel fighters now acting as prison guards, said.
"We now have the army outside the prison."
At Sabha hospital, doctors tell of how patients have been shot inside the grounds by angry rival tribesmen seeking revenge. Bullet holes are still visible on the floor.
Restoring order in the south is important to the stability of the wider region, where Islamist influence is spreading after the defeat of the insurgents in Mali.
A string of attacks in Niger including on a French-run uranium mine have shown how rebels have taken advantage of a security vacuum since the Mali conflict.
Security officials say lawless southern Libya has become the latest haven for Islamist groups. Paris has put the blame firmly on these groups for attacking its embassy in Tripoli in April.
Libyan officials insist Islamists have not found shelter in their deserts.
"There are no al Qaeda groups here. We can say that and we know," said Mahmoud Abdelkareem, an official from Obari council involved in security matters for the south. "Our men in the desert would find them easily and this has not happened."
But Western nations are worried. Earlier this month NATO, which played a major role in toppling Gaddafi, said it would send experts to Libya to see how it can improve security.
"We can't deny some activities are going on. The fact that the area is not properly secured encourages smuggling, perhaps even training camps," said one Libyan security official from the town of Ghadames, on the border with Algeria.
Residents in Sabha tell of hearing stories of weapons being sold across the border and areas briefly shutdown by militias.
"There are people who went to fight in Mali and others have come from there. But they are keeping a low profile, most likely near the borders," said the first local government official.
"Any cooperation however between a tribal group here and them is likely to be financial rather than ideological."
Gaddafi's overthrow flooded the Sahara with pillaged weapons and ammunition, which Tripoli has failed to clamp down on.
"Libya is an open air arms market; it will remain a source of weaponry for 10 years," an Algerian security analyst said.
Security sources say veteran al Qaeda commander Mokhtar Belmokhtar acquired arms in southern Libya and his fighters used it as a transit route before a mass hostage-taking at a gas plant in Algeria in January in which dozens were killed.
Many fear Libya's oil facilities, also guarded by former rebels, may face a similar threat.
"The situation in the south has worsened dangerously fast," Muftah Bukhalil, head of the intelligence office in Kufra, said.
"You can just about expect anything these days."
Additional reporting by Ghaith Shennib in Kufra and Myra Macdonald in Algiers; Editing by Peter Graff