They 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.

Pro-Gaddafi forces are believed to have more than 200 Scuds in their arsenal.

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

On August 15 they advanced into two strategically important towns controlling access to the capital, from the west and the south.

Fighting was reported in both Zawiya, 50km (30 miles) west of Tripoli, and Gharyan, 80km (50 miles) to the south.

Col Gaddafi's forces are still thought to control the Zawiya oil refinery - the only one in western Libya - where they get most of their fuel.

Earlier, the rebels said they had seized Gharyan - a town in the Nafusa Mountains that straddles the road connecting Tripoli with Sabha, Sorman - west along the road from Zawiya to Tunisia - and Tiji, although this has not been confirmed by residents.

If the towns fell to the rebels, they would have Tripoli surrounded by land, with Nato blocking sea access.