Earlier this week warplanes bombed IS positions in Sirte, the most significant IS stronghold outside Iraq and Syria, while naval forces fired missiles into the port, officials said. The offensive continues, BBC reported.
The forces, aligned to the UN-backed unity government in Tripoli, began the battle to re-take the city last month.
Their spokesman, General Muhammad al-Ghusri, said senior IS leaders had fled into the desert to the south, but many militants were still under siege in the city centre.
Clashes centred on the Ougadougou conference centre, which was once a venue for international summits but has now become an IS command centre.
Forces loyal to the government targeted the conference centre with heavy artillery fire, backed by warplanes.
The US said the unity government should be allowed to arm itself against IS. Secretary of State John Kerry said this would be “the only way to generate the cohesion necessary” to defeat the militants.