The National Transitional Council (NTC) fighters say they had to retreat in part of the town on Thursday under heavy artillery fire.
NTC officials also backtracked on reports that one of Gadhafi's sons was arrested in Sirte.
On Wednesday, some NTC officials said the former leader's son, Mutassim, had been detained and taken to the eastern city of Benghazi for questioning. But provisional government officials later said the reports could not be confirmed.
NTC officials say revolutionary forces did capture some fighters believed to be with Mutassim in Sirte. Mutassim was Libya's national security adviser and had a strong role in the military and security forces under his father's government.
There have been false reports regarding the capture of senior Gadhafi figures in the past, including that of another son, Saif al-Islam, who is still on the run.
Meanwhile, Amnesty International is urging the NTC to end arbitrary detentions and "widespread abuse" of detainees, saying there is a risk of repeating patterns of Gadhafi's rule.
In a report released Thursday, the London-based rights group said it found a pattern of beatings and ill-treatment of pro-Gadhafi soldiers, suspected loyalists and alleged mercenaries in western Libya.
The group said its findings are based on interviews of about 300 prisoners during August and September.