Two major armed groups in eastern Libya announced their support Saturday for the UN-backed national unity government as it struggles to establish its authority in the east of the country.

The Government of National Accord (GNA), established in Tripoli more than two months ago, has been trying to unify violence-ridden Libya and exert its control over the entire North African country.

However, it faces opposition from a competing authority based in the east which has its own armed forces -- militias and some units of the national army -- commanded by controversial General Khalifa Haftar.

Two major groups formerly loyal to Haftar, the special anti-terrorist force and a military intelligence brigade -- decided to throw their lot in with the GNA.

On Saturday, the commanders of the two units held a joint press conference with GNA defence minister-designate Al-Mahdi al-Bargathi to announce their decision.

On the ground, meanwhile, forces loyal to the GNA said they had retaken an air base near the coastal city of Sirte, bastion of the Islamic State (IS) jihadist group.

The pro-government forces announced on Facebook they had "retaken control of Al-Gordabiya air base", 15 kilometres (nine miles) south of Sirte, the hometown of the late dictator Moamer Kadhafi.

It said the GNA forces were able to take the complex with the backing of "five air raids against jihadists and their equipment".

GNA forces also announced the "liberation from the hands of Daesh" (an Arabic acronym for IS) of the town of Abu Hadi, 15 kilometres southeast of Sirte.

The unity government's forces and those of the rival authority in the east are currently engaged in a race to be the first to drive IS out of Sirte.

The international community believes this could jeopardise efforts to defeat IS, whose threat has grown since they established a foothold in Libya at the end of 2014.

On Tuesday, the UN's special envoy on Libya, Martin Kobler, had called on all the country's armed factions to unite against the jihadist group.

He said the rival administrations that have established themselves since the fall of Kadhafi in 2011 should "unite their efforts" into a single army.

Led by premier-designate Fayez al-Sarraj Fayez and internationally recognised, the GNA has already received the support of former authorities who controlled the capital and armed militias in the west.

The Libyan Central Bank and the State Oil Company have also rallied to his authority.

But the unity administration has yet to receive a vote of confidence in parliament, which remains linked to the former internationally recognised government also based in the east.