The offshoot of the Islamic State (Isis) in Libya – the militant group's largest franchise outside the Middle East – has continued to lose crucial ground to advancing pro- government militias in its Mediterranean stronghold of Sirte.

Militias – predominantly from the central Libyan city of Misrata – announced they had seized Sirte's electricity company headquarters, TV and radio building and the Bin Hamel mosque. Two of the city's key districts have also been recaptured by the brigades, which are aligned with Libya's Government of National Accord.

Rida Issa, a spokesman for the military control room overseeing the western advance into Sirte, told Reuters 16 pro-government fighters had been killed and 60 wounded in clashes on Tuesday (21 June).

A foreign media spokesman in Tripoli said that the city's 700 district had been declared safe, following its high buildings being cleared of Isis militants. The group has used snipers and an onslaught of car bombs in an attempt to halt the government forces' advance.

Isis seized control of Sirte, a former Gaddafi heartland prior to the country's 2011 revolutionary war, last year. Thousands of Isis fighters, many of them foreigners from Tunisia, the Levant and sub-Saharan Africa, were said to have travelled to the city.

Militias linked to Libya's UN-backed Government of National Accord began their advance into Sirte in May. They seemed to have made slow progress prior to a breakthrough on 9 June, during which dozens of pro-government fighters were killed and hundreds wounded.

The Misratan forces seized control of a vital air base, several military camps and a central roundabout where IS militants had hung the bodies of executed enemies for intimidation purposes. At the time military leaders said the city would be recaptured in a matter of days rather than weeks.