The news is a major blow to the unrecognised authority in Tripoli that is refusing to give up power.
The announcement came in a statement on the official Facebook page of the Sabratha municipality yesterday, after a meeting between representatives from the 10 coastal cities in the west of Libya located between Tripoli and the border with Tunisia.
The statement called on all Libyans to "support the national unity government" and welcomed the arrival of prime minister-designate Fayez al-Sarraj along with several members of his cabinet to the capital.
The non-recognised government of Tripoli have branded the arrival of UN-backed Sarraj illegal, demanding he leave or surrender.
Libya has had two administrations since mid-2014 when the militia alliance overran Tripoli, setting up its own authority and forcing the internationally recognised parliament to flee to the country's remote east.
The country was plunged into chaos and fighting between rival military factions following the death of dictator Moamer Kadhafi in 2011, killed following a Western-backed military intervention in the country.