Forces loyal to Muammar Gaddafi fired a Scud missile for the first time in the country's civil war, a US defense official said, after rebel advances left the Libyan leader isolated in his capital.

Rebels fighting to end Gaddafi's 41-year rule seized two strategic towns near Tripoli over the past 24 hours, cutting the city off from its supply lines and leaving the Libyan leader with a dwindling set of options if he is to stay in power.

A doctor restrains a man believed to be a Gaddafi army soldier as he is treated for a gunshot wound at the Bir Muammar Hospital on the outskirts of Zawiyah on Monday. Rebels who attacked the strategic town of Zawiyah on Saturday lost at least six men on Monday. [Reuters]

The Scud missile was fired on Sunday morning from a location about 80 km east of Sirte, Gaddafi's home town, and landed east of the coastal oil town of Brega where rebels are fighting for control, the official said.

The missile came down in the desert, injuring no one, said the official, who was speaking on the condition of anonymity. There was no immediate comment from the government in Tripoli.

In the six months of fighting up to now, Gaddafi's forces have been using short-range Grad rockets but have not before deployed Scud missiles, which have an estimated range of about 300 km.

The government in Tripoli has stocks of Scud missiles which were acquired from the Soviet Union in the 1970s, and some bought from Democratic People's Republic of Korea, according to online defense forum globalsecurity.org.

It said many of Libya's missile systems "are old and likely are suffering from maintenance problems".

Analysts say the rebels' strategy now is to isolate the capital and hope the government will collapse, but they say it is possible too that Gaddafi will opt to stage a last-ditch fight for the capital. In a barely audible telephone call to state television in the early hours of Monday morning, Gaddafi called on his followers to liberate Libya from rebels and their NATO supporters.

"Get ready for the fight ... The blood of martyrs is fuel for the battlefield," he said.

Rebel push

He was speaking as rebels made their most dramatic advances in months of fighting, shifting the momentum in a conflict that had been largely static for months and was testing the patience of NATO powers anxious for a swift outcome.

Rebel forces in the Western Mountains south of Tripoli surged forward last weekend to enter Zawiyah. The town is about 50 km west of Tripoli and, crucially, straddles the main highway linking the capital to Tunisia.

A day later, rebels said they had captured the town of Garyan, which controls the highway leading south from Tripoli and linking it to Sabha, a Gaddafi stronghold deep in the desert.

"Gaddafi has been isolated. He has been cut off from the outside world," said a rebel spokesman named Abdulrahman from the Western Mountains.

Early on Tuesday, rebels on the outskirts of Zawiyah said forces loyal to Gaddafi were still on the eastern edge of the town, from where they have been attacking with mortars, Grad rockets and sniper fire.