Fighters of Libya's ruling National Transitional Council (NTC) on Sunday retreated from several positions in the northern coastal town of Sirte, so as to allow NATO to bomb sites that are holding up the remaining forces of the previous leadership.

Libyan National Transitional Council (NTC) fighters fire at supporters of fallen leader Muammar Gaddafi during street battles in the city of Sirte on October 15, 2011. [Xinhua]

According to a source close to the NTC, their fighters fought fierce battles on Sunday with the diehard loyalists of fallen leader Muammar Gaddafi at the Dollar district and the so-called Area Two of Sirte, hometown of the country's 42-year-long ruler.

The battles forced the NTC fighters to pull back, which will give way to NATO's aircraft and heavy artilleries aiming at hideouts of the pro-Gaddafi forces, said the source, who asked to remain anonymous.

Meanwhile, a military source in Benghazi, Libya 's second largest city, was quoted as saying by local newspaper Cyrene that the NTC has continued to besiege Sirte on all sides, and advanced several kilometers inside the Clio neighborhood.

Moreover, the source said that Mutassim, Gaddafi's fourth son, might be still in Sirte. Conflicting information occurred in the previous days on the capture of Mutassim in Sirte, where prolonged battles have killed tens of NTC fighters.

On Saturday, the NTC moderated offensives on Sirte to secure the exits of civilians still trapped inside the city where an estimated 400 pro-Gaddafi soldiers and mercenaries, with few heavy weapons but snipers and some RPGs, were making a gamble final fight.

NTC leaders have repeatedly vowed to take full control of Sirte in a timely fashion, but fierce resistance from pro-Gaddafi forces makes the prospect of the coastal town still misty.