Tripoli (AFP) - Violent clashes raged Tuesday south of Tripoli, marking an uptick in fighting as the UN envoy for Libya warned of a "long and bloody war".

Heavy artillery fire could be heard from the city centre as the most intense fighting took place since May 6, when the holy Muslim month of Ramadan started.

More than six weeks since commander Khalifa Haftar launched an assault on the capital, fighters backing the UN-recognised Government of National Accord (GNA) made advances in the southern Salaheddin neighbourhood.

AFP journalists on the front lines estimated pro-GNA forces progressed by two or three kilometres (up to two miles) in the residential district.

Moustafa al-Mejii, a GNA spokesman, said fighter planes supported ground forces in their advance, carrying out strikes against tanks and heavy weapons in a barracks.

Haftar, who is backed by Egypt and the United Arab Emirates, launched a lightning assault on Tripoli with his self-styled Libyan National Army (LNA) on April 4.

But forces backing the GNA have put up a fierce resistance and the LNA has been held back in Tripoli's southern suburbs.

Weeks of fighting have killed 510 people and wounded 2,467, according to the latest toll from the World Health Organization.

More than 75,000 people have fled their homes, according to the United Nations, while 100,000 are trapped by the conflict.

- 'Long and bloody war' -

On Tuesday the UN's envoy for Libya, Ghassan Salame, warned greater efforts were needed to stem the flow of arms to the North African country.

"The violence on the outskirts of Tripoli is just the start of a long and bloody war on the southern shores of the Mediterranean, imperilling the security of Libya's immediate neighbours and the wider Mediterranean region," he told the UN Security Council.

Without immediate action by the international community, he said, "Libya will descend into civil war which could potentially lead to a Hobbesian all-against-all state of chaos or partition of the country."

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His appeal came after the Tripoli-based Government of National Accord posted photographs at the weekend of dozens of Turkish-made armoured vehicles that it said on its Facebook page were fresh deliveries for its fighters.

Pro-Haftar websites also posted photos and video footage of Jordanian-built armoured cars they said were supplied to the LNA.

Haftar advanced from his stronghold of eastern Libya with an offensive on the south of the country earlier this year, before turning his forces towards the capital.

His forces control Libya's most important oil fields, but the Tripoli-based National Oil Corporation manages production and revenues are chanelled through the central bank in the capital.

Libya has been mired in chaos since NATO-backed forces deposed and killed former dictator Moamer Kadhafi in 2011, with militias and tribes vying for control of the country's resources.