US defense officials say pro-Gaddafi forces have fired a Scud missile for the first time in the country’s six-month conflict.

The officials say it was launched from near the coastal city of Sirte and aimed at Brega – currently held by rebels – but landed harmlessly in the desert.

Forces loyal to Gaddafi are believed to have more than 200 Scuds in their arsenal.

The move comes as rebels continue to push towards the capital, Tripoli.

On Monday the rebels advanced into two strategically important towns controlling access to the capital, from the west and the south.

Fighting has been reported in both Zawiya, 50km west of Tripoli and Gharyan, 80 km to the south.

Medics outside Zawiya told Reuters news agency that sniper and mortar fire from government troops had left three civilians dead.

The Scud launch was detected by US forces who say the missile was launched on Sunday morning and landed in the desert about 80 km outside the strategic port of Brega.

There was however no word from Libyan authorities.

Scud missiles have a range of up to about 500 km and can carry a warhead of up to about one tone.