Libyan forces of the National Transitional Council (NTC) Friday launched their biggest assault on Sirte, one of the heavily-armed towns still under the control of fallen leader Muammar Gaddafi, Reuters reported.

Heavy smoke rises from Sirte city after NATO bombing of the positions of Gaddafi loyalists during heavy fighting with anti-Gaddafi forces, as they push forward towards the centre of Sirte October 7, 2011. [Agencies]

The attacks, launched from the east and west, are aimed at seizing the center of the Gaddafi hometown, about 450 km east of Tripoli, the source told Xinhua on condition of anonymity.

At least 12 people were killed and more than 190 injured, doctors said.

NTC forces said that the battle is entering its final and decisive phase. But NTC commander Mustafa Al-Ameen said while progress was being made, there was still resistance from Gaddafi loyalists, according to Reuters reports.

"The forces that went in managed to advance into Sirte but we couldn't go further because of sniper fire," he said.

What hinders progress are still the civilians and the spread of pro-Gaddafi snipers on the building roofs across the town, who shot everyone that moves in the city indiscriminately, Xinhua quoted the anonymous source as saying. 

The prolonged struggle with the Gaddafi loyalists has prevented the NTC to establish an effective government and rebuild oil production vital to its economy.

Plumes of smoke are seen while heavy explosions are heard from inside Sirte on Friday, as the weeks-long seesaw battles there have put the town in dire need of drinking water, food, electricity and fuel. 

Thousands of civilians have fled Sirte as fighting has intensified, describing increasingly desperate conditions for those inside the city, according to Reuters reports.

NTC trucks with ammunition and artillery batteries were brought forward to the eastern front line, 1.2 km (0.8 miles) from the city centre.

But Gaddafi loyalists used sniper fire and rocket-propelled grenades to prevent NTC forces from entering the city centre.  

Anti-Gaddafi forces seized control of virtually all of Libya nearly two months ago, overthrowing the Gaddafi regime.

The former Libyan leader fled Tripoli when the capital was seized by the NTC and his whereabouts remain unknown while several of his family members are in hiding or have fled the country.

But he is not thought to be in Sirte, which has been under siege for weeks, BBC reported.

A senior U.N. official in Libya, Ian Martin, urged NTC fighters not to aggravate tensions by carrying out g violent revenge against Gaddafi supporters in Sirte.

"We are expressing our concern that the situation ends in a way that lays the foundations for national reconciliation rather than exacerbates the problems that a new government will face," Martin told Reuters in an interview. 

"I appeal to all to respect the calls made by the National Transitional Council (NTC) that there should be no revenge even against those responsible for war crimes and other grave violations," Martin said in a press release.