Despite a UN embargo, weapons are still flowing into Libya where an assault on the capital by strongman Khalifa Haftar threatens to escalate into a proxy war between regional powers.
The UN envoy for Libya has warned that the battle for Tripoli is "just the start of a long and bloody war".
Ghassan Salame is calling for immediate steps to cut off arms flows fuelling the fighting.
One side of the conflict said it is receiving armoured vehicles from Turkey, while the other said it is receiving them from Jordan.
More than 75,000 people have fled their homes in the latest fighting, according to the World Health Organisation.
At least 510 people have been killed, WHO figures show.
Addressing the UN Security Council, Mr Salame warned that the conflict could descend into what he called an "all-against-all" state of chaos.
"I am no Cassandra, but the violence on the outskirts of [the capital] 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," Mr Salame said.
Haftar, whose self-styled Libyan National Army (LNA) is allied with an administration in eastern Libya, is supported especially by Egypt and the United Arab Emirates (UAE).
On the other side, Turkey and Qatar back the Tripoli-based Government of National Accord (GNA) which is recognised by the international community.
Haftar on April 4 launched an attack on the capital that has ground to a halt on the southern outskirts of Tripoli in the face of GNA forces backed by militias from western Libya.
Both sides have called in reinforcements as fresh weaponry arrives despite a UN arms embargo officially still in place since a 2011 revolt that toppled Libya's longtime leader Muammar Gadhafi.
The GNA boasted on Saturday of new "armour, ammunition and... weapons" for its fighters.
It did not specify the source but posted photographs on its Facebook page of dozens of Turkish BMC Kirpi armoured vehicles at Tripoli port.
The Moldova-flagged vessel which made the delivery belongs to a Turkish firm and set sail from a port in Turkey, according to navigation websites.
Arnaud Delalande, a defence consultant and specialist on Libya, interpreted the delivery as an apparent open show of support for the GNA.
In a swift response, pro-Haftar websites on Sunday posted photos and videos of Jordanian-built armoured cars they said were being supplied to the LNA.
Such deliveries "show that neither party plans to give in and that it is headed more towards a war of attrition", said Delalande.
A military source in eastern Libya, contacted by AFP, declined to confirm or deny the delivery of Jordanian armour but said "there is no halt to the flow of reinforcements" to the front line.
On the GNA side, Turkish support "will help narrow the gap in weaponry between the two sides", said Wolfram Lacher of the German Institute for International and Security Affairs (SWP) in Berlin.
He said: "The large number of UAE-made armoured vehicles had helped Haftar's forces in suburban areas, and they could now lose that advantage."
However, "the risk is that such overt support (from Turkey) will prompt Haftar's backers to step up their assistance, and perhaps to intervene even more directly."
For Lacher, "this war is now turning into a proxy war between rival Middle Eastern powers".
"The more both sides receive arms and ammunition from their foreign backers, the longer the war will last, the more destructive it will be, and the more difficult it will be to resolve," he said.
The more than six-week-old battle for Tripoli has already cost over 450 lives and left 2,000 injured and displaced almost 70,000 residents, according to UN agencies.
Both sides have ignored international calls for a ceasefire and dialogue.
The front lines are largely frozen, although "the balance of power on the ground is in favour of the pro-GNA forces but not decisively", said Delalande.
In the air battle, the rival forces are roughly even with around 15 fighter-bombers on each side, he said.
But increased air support from the UAE, especially through the Chinese-built Wing Loong drones deployed in eastern Libya since 2016, could swing the balance of power in Haftar's favour, he said.
According to a report seen by AFP, UN experts are probing the UAE's possible military involvement in Libya following Blue Arrow missile strikes in April by Chinese-built drones of the type used by the Emirati military.
The UN's group of experts on the country noted in a September report an increase in the number of armoured vehicles being supplied to the LNA as well as mortars and rocket launchers.
Libyan analyst Jalal al-Fitouri said "secret or public imports of arms have been going on for years" but have now been stepped up by both sides.