"Three explosions from cars driven by Islamic State suicide bombers targeted our forces in Sirte," Rida Issa, a spokesman for the unity government's forces, told AFP.
"There are several wounded among our forces," he said, without providing further details.
One bomber blew up his car a few metres from a group of pro-government fighters near the Abu Hadi roundabout in the city's southeast, Issa said.
The other bombers targeted pro-government forces and a field hospital in the city's west.
The loss of Sirte, former dictator Moamer Kadhafi's home town, would be a major blow to IS at a time when it is under mounting pressure in Syria and Iraq.