The UN peace envoy for Libya, Bernardino Leon, said the latest round of peace talks launched in Morocco on Thursday could prove to be the "moment of truth" for his efforts to persuade rival parties to sign up to an agreement to form a unity government.

"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," Leon told reporters in Skhirat, the beach resort near Rabat, the Moroccan capital, where the talks are taking place.

"We are very hopeful that they will understand that this deadline of 20 September must be the last one, must be the one that will allow Libya to get out of this crisis."

Libya, which has spiralled into violence since former leader Muammar Gaddafi was toppled and killed in 2011, has two rival administrations - a body in Tripoli known as the General National Congress (GNC) and an elected but contested government based in Tobruk in the country's east. Fighters allied to the Islamic State group have also increased their strength in recent months thanks to the ongonig political turbulence.

According to Leon on Thursday, Islamic State 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".

The bank was stormed by gunmen back in 2013 when they made off with $54mn, Libyan officials told media.

Leon's peace plan calls for an accord to be reached by 20 September, ahead of a meeting of the United Nations General Assembly, with the new 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.

Previous talks have failed, with either the GNC or Tobruk saying that the proposals unfairly favoured the other side.