The UN peace envoy for Libya, Bernardino Leon, said the latest round of peace talks launched in Morocco could prove to be the "moment of truth".
"We are starting a new round of talks in the Libya process, which we hope will be the final round, the moment of truth for the parties," he said in Skhirat near the Moroccan capital on Thursday.
"We are very hopeful that they will understand that this deadline of 20th of September must be the last one, must be the one that will allow Libya to get out of this crisis."
Libya, torn apart since dictator Muammar Gaddafi's ouster in 2011, has two rival administrations - a body in Tripoli known as the General National Congress (GNC) and an internationally recognised government based in Tobruk in the far east.
Leon's peace plan calls for an accord to be reached by September 20, ahead of the United Nations General Assembly, with the government coming into force a month later.
A two-year transitional government would work towards organising parliamentary elections, under the terms being discussed.
Leon said a deal on creating a unity government could be reached "in the coming days", but that difficult work remained before a final accord.
The rising influence of the Islamic State of Iraq and the Levant (ISIL) group in Libya and the country's emergence as a smuggling hub for migrants risking their lives to cross the Mediterranean have added urgency to the long-running and often derailed UN talks.
According to Leon on Thursday, ISIL fighters had "in the last hours" taken control of the offices of the Libyan central bank and several private banks in the coastal city of Sirte, calling it a "very concerning and very serious development".