Ghassan Salame, the UN's special envoy to Libya, on Tuesday warned that the country was facing a protracted civil war and he blamed Field Marshal Khalifa Haftar's recent offensive for allowing ISIS to infiltrate southern areas.
In his first briefing to the Security Council since the crisis began last month, Mr Salame also criticised nations involved in what he said were flagrant breaches of an arms embargo.
Weapons have been supplied to both sides in the conflict since the latest violence broke out.
Mr Salame received strong backing from the council, with Britain's permanent representative to the UN Karen Pierce describing the envoy's assessment as “sobering and alarming”.
Ms Pierce called for an immediate ceasefire, saying there would never be “a clear victory for any party in Libya” as long as arms continued to flood in.
The top US representative to the UN, Jonathan Cohen, said instability in the capital could only be stopped by a ceasefire, and he urged “all sides to avoid escalation”.
Field Marshal Haftar's Libyan National Army, based in the east of the country, began its offensive on Tripoli on April 4, leading to deadly clashes with militias loyal to the UN-backed Government of National Accord.
More than 460 people, including 29 civilians, have been killed, 2,400 wounded and 100,000 men, women and children remain trapped on front lines, Mr Salame said.
“This is the report whose delivery I have spent nearly the past two years trying to avoid,” he told the council.
“Forty-eight days into the attack on Tripoli by Gen Haftar's forces there has already been too much death and destruction.”
The unexpected offensive began while UN Secretary General Antonio Guterres was visiting Libya barely a week before a planned conference in the city of Ghadames.
The conference was intended to organise national elections and unification of the country's rival governments.
“To see those who had enthusiastically taken up our invitation to Ghadames suddenly take up arms against each other to attack the capital, or to defend it, has thrown me into the deepest level of sadness for the opportunity lost and for a hope killed exactly 10 days before its realisation,” Mr Salame said.
The fighting was likely to have effects far beyond the capital, he said.
“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 wider Mediterranean region,” Mr Salame said.
Militants are already looking to exploit the situation.
“The security vacuum created by the withdrawal of Gen Haftar's troops from the south, coupled with the focus of the western forces on the defence of the capital, is already being exploited by Daesh and Al Qaeda," Mr Salame said.
"In the south of Libya, the black flags of Daesh are appearing and I am dismayed to report that since April 4, there have been four separate Daesh attacks.”
Kuwait's permanent representative to the UN, Mansour Al Otaibi, the current Arab member of the Security Council, also called for an end to fighting.
“We were surprised by the attacks on Tripoli in the first week of April,” Mr Al Otaibi said.
“There is never going to be a military solution. We urge all parties to exercise restraint and respond positively to calls from the UN mission for there to be a ceasefire.”
