Addressing the Security Council on Tuesday, Ghassan Salame said many countries were supplying weapons to the UN-recognised Government of National Accord (GNA) in Tripoli and forces led by renegade general Khalifa Haftar.
The commander, who is allied to a rival administration in Libya's east, launched an offensive to seize the capital on April 4, but his forces have been bogged down in the southern outskirts of Tripoli.
"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," Salame said.
Without immediate measures to stop the flow of arms, "Libya will descend into civil war which could potentially lead to a Hobbesian all-against-all state of chaos or partition of the country," he said.
Salame's appeal came after the GNA posted photographs over 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 Haftar's Libyan National Army (LNA).
The fighting has threatened to ignite a civil war on the scale of the 2011 uprising that toppled and killed longtime ruler Muammar Gaddafi. The UN Security Council imposed an open-ended arms embargo on Libya in February of the same year.
More than 75,000 people have been driven from their homes in the latest fighting and 510 have been killed, according to the World Health Organization.
'In a confidential report to the Security Council earlier this month, UN experts said missiles fired at pro-Tripoli forces in April pointed to a likely drone attack that could involve a "third party," possibly the United Arab Emirates. Haftar is backed by Egypt and the UAE.
In recent weeks, Salame said there has been an average of five night-time raids on GNA positions by "unknown aircraft". For the residents of Tripoli, Ramadan nights "normally convivially spent in the company of one's neighbours and family have become periods of sheer terror".
Without international action, the fighting in Libya could "lead to a permanent division of the country", Salame warned, adding that armed groups, including the Islamic State of Iraq and the Levant (ISIL, or ISIS) and al-Qaeda have already exploited the security vacuum created by the battle for the capital.