28 September 2011 Last updated at 19:47 ET

Gaddafi loyalists in Sirte thwart Libya NTC advance

By Alastair Leithead BBC News, Sirte, Libya

For months, those who began the revolution have advanced west across Libya taking town after town for the National Transitional Council (NTC), but here in Sirte they are facing a huge challenge.

As Col Gaddafi's home town - seat of his family and tribe - it has always had preferential treatment and now it is being fiercely defended.

The rebel army grew in experience and hardware as it followed the coast road across the country, learning the skills to fight a professional army and relying on Nato to ensure it was an uneven battle.

From a distant vantage point, you hear the roar of the jet, then see the plume of smoke and finally, a few seconds later, the crunch of the explosion reaches you.

Nato aircraft have been bombing Sirte for a day and two nights, striking pro-Gaddafi positions in and around buildings of the new university and the city centre.

Ambulances race past

But still the new interim government's army is held back by accurate rocket fire and snipers - prevented, as yet, from taking the whole of this hugely symbolic city.

The fire coming from the middle of the city is very accurate, and it pins down those trying to bring this siege to a quicker end.

After dark, perhaps a dozen ambulances headed east to the hospitals, carrying the dead and the injured from another day of war.

The pro-Gaddafi forces have some very modern weapons at their disposal and despite Nato destroying what is thought to have been an arms dump, which burned for two days, they are believed to have plenty of ammunition and supplies.

The celebratory gunfire when the once-rebels reached the gate to central Sirte after weeks in the desert was soon turned on to the defenders of the city as front lines were quickly established.

There are tens of thousands of civilians thought to be still in Sirte and despite reassurances of their safety, both sides have been exchanging rockets and artillery shells.

All roads in and out are now front lines, and at the coast, the port and a big hotel were captured by the former rebels.

But Gaddafi's loyalists counter-attacked - at one stage, there were gun battles raging over just a few hundred metres.

Mercenaries from elsewhere in Africa are blamed for defending what many believe is now no longer defendable.

Locals talk of those from Mauritania, invited to move to Libya 40 years ago and to become members of Col Gaddafi's small tribe.

There have been sightings of one of his sons in Sirte. If those defending the city feel they have nothing to lose, they may fight to the death street by street.

The NTC tactics may have to change and adapt.

It was much easier in the wide open spaces of the desert to use the aerial advantage Nato gave them, but the tall buildings, narrow streets and civilians in the city itself will prevent air power alone from winning this battle.

They also want to keep the number of casualties down. Their ranks are filled by engineers or teachers and businessmen - all crucial for bringing the country back after revolution and war.

As the bombs continue to fall and the ambulances race past, all while little progress is made; it is easy to see how this siege could rumble on for perhaps even weeks.

But then it could all crumble in a day, as other cities have, with a rising from within - but this is clearly Gaddafi country and many in this city of privilege have a great deal to lose under the new administration.