Forces in Libya have battled to a deadlock near Tripoli, UN envoy Ghassan Salame said on Thursday.
Mr Salame said a unified call from the international community was needed to end the fighting.
"After the first successes of the Libyan National Army two weeks ago, we are witnessing a military deadlock," he said.
Mr Salame said international divisions had encouraged Field Marshal Khalifa Haftar to pursue his assault on Tripoli.
He has been backed by Russia and France, which have prevented an EU call for Field Marshal Haftar's Libyan National Army to halt the offensive.
The LNA commander pledged to end terrorist and militia rule in Tripoli and restore order.
Libya has experienced factional anarchy since dictator Muammar Qaddafi fell to a western-backed uprising in 2011.
Meanwhile, LNA troops repelled an attack by an armed group on a major airbase in the country’s south on Thursday.
Army officials said their soldiers had repelled the attack on Tamanhint airbase near the city of Sabha.
They said a soldier was killed at the base’s main gate in the attack out by a militia.
It was unclear whether the attackers were from militias defending the internationally recognised government in Tripoli, which is fighting Field Marshal Haftar’s force.
The LNA is allied to a parallel government based in the eastern city of Benghazi.
The fighting has killed at least 205 people, including 18 civilians, and wounded 913, the World Health Organisation said on Thursday.
Field Marshal Haftar has concentrated his forces near Tripoli, mainly using the Jufrah airbase but also Tamanhint to fly in supplies and send them north by road towards the Mediterranean.
The LNA this year seized Libya's sprawling south with its two main oilfields, although tribesmen with flexible loyalties remain strong in the desert region.
At the weekend the LNA sent units to the eastern oil ports of Ras Lanuf and Es Sider to defend any attack on them.
The conflict in Libya threatens to disrupt oilflow, increase migration across the Mediterranean to Europe and allow militants to exploit the chaos.
International powers are angry that the flare-up in Libya has scuttled a UN peace process, but have been unable to present a united front to defuse the crisis.
The UAE issued a statement with European countries expressing concern about the fighting and urging all parties to stand down.
France has given Field Marshal Haftar support in the past, considering him the best chance to end Libya's disorder.
Italy, the former colonial power with extensive oil holdings in Libya, backs the Tripoli government of Prime Minister Fayez Al Sarraj and was angered by French reluctance to endorse a recent EU resolution urging Field Marshal Haftar to halt his advance.
The UN Security Council has been considering a resolution drafted by Britain that would demand a ceasefire and urge all nations with influence over the parties to ensure compliance.