Losing the city would compound the setbacks the jihadist group has suffered in Syria and Iraq.
They have struggled to defend themselves against suicide bombings, sniper fire and landmines.
More than 350 brigade fighters have been killed and at least 1,500 wounded since the campaign to recapture Sirte began...
Losing Sirte would be a major setback for the militants, already under pressure in Syria and Iraq.
Discussion of a 5,000-strong Italian-led peace keeping mission has fizzled out, and foreign ground intervention has been limited...
At least 20 members of the government-backed forces were killed and 120 wounded in fighting in Sirte on Friday
IS militants beheaded 12 local militiamen who had been battling them in Sirte