Zawiya: Libya's rebels threatened to isolate Tripoli by blocking key supply routes and cutting oil pipelines after a dramatic weekend advance put them in the strongest position since the 6-month-old civil war began to attack Muammar Gaddafi's stronghold.

In Washington, the Obama administration said on Monday the US was encouraged by the rebel advances and hoped they had broken a months-long stalemate with Gaddafi's forces.

"We are closing the roads for Gaddafi so there is no way for him to bring anything to Tripoli," a rebel field commander, Jumma Dardira, said media.

The rebels' push into the strategic city of Zawiya on Saturday brought them within 30 miles (48 kilometers) of Tripoli, the closest they have ever gotten.

Also on Monday, US defense officials said Libyan government forces tapped into their stores of Scud missiles this weekend, firing one for the first time in this year's conflict with rebels, but hurting no one.

The missile launch was detected by US forces shortly after midnight Sunday and the Scud landed in the desert about 50 miles (80 kilometers) outside Brega, said one official, who spoke on condition of anonymity to discuss military operations.

Rebel and regime forces have battled over the strategic port city of Brega throughout the conflict, and control has swung back and forth between the two sides.

According to the military, the Scud missile was launched from a location about 50 miles (80 kilometers) east of Sirte, a city on the Mediterranean coast about 230 miles (370 kilometers) east of Tripoli. Noting that Scuds are not precision guided missiles, officials said they couldn't tell if Brega was the target.

Early in the conflict, NATO and US forces targeted sites around the country where Gaddafi stored surface-to-surface missiles like Scuds, largely because they worried that he would use them to target areas beyond his control.