Libyan National Transitional Council (NTC) fighters are regrouping for another push towards Sirte, the birthplace of toppled leader Muammar Gaddafi.

On Monday night battles raged around Sirte, as NTC fighters and Gaddafi forces traded gunfire and rockets.

An NTC commander on Tuesday told Al Jazeera's Hoda Abdel Hamid that they had reached the eastern gate of Sirte on Monday evening, but had to pull back after intense clashes with Gaddafi loyalists.

"They [NTC fighters] say they are hoping to keep the momentum, over the past few days, they have advanced a good 40km towards Sirte but the closer they get, the tougher it gets," she said.

Sirte, about 400km southeast of the capital, Tripoli, is one of the last remaining strongholds of Gaddafi.

Most of the NTC fighters besieging Sirte are from Misrata, which survived a brutal weeks-long siege by Gaddafi forces during the early days of the revolution.

NATO radio announcements have been repeatedly urging Gaddafi supporters in Sirte to surrender.

"The world is watching you," the announcement said. "This is your opportunity to side with the Libyan people and choose the right side of history."

Situation 'precarious'

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Residents who have fled Sirte told Al Jazeera that hundreds of people were still trapped in the town.

"There's no electricity, no phone coverage, nothing," resident Ibrahim Ramadan told the Reuters news agency, as he fled the fighting.

An NTC commander told the AFP news agency that his forces were concerned about the town's civilians.

A number of people, including children, were seen taking refuge at a mosque on the outskirts of Sirte as continuous gunfire was heard in the distance.

"The situation in Sirte is very precarious," said one man fleeing Sirte with his family.

Audio message

Gaddafi's whereabouts are not known since NTC fighters took over Tripoli and much of the country, ending his more than 40-year-old rule.

But in a televised audio message on Tuesday, a voice purportedly of Gaddafi said it was impossible to overthrow the political system in Libya.

The message said claims that Gaddafi had been toppled were "ludicrous [and] a mockery".

"The political system of Libya is built on the authority of the people; it is a system exercised by all the Libyan people, men and women, in the people’s congresses and committees," said the message.

"It is impossible to topple his unique system. Do not believe that the regime is toppled by the sea and air bombardment. This is a farce, a charade; and the NATO bombardments will not last."

Source: Al Jazeera and agencies