Three weeks ago anti-Gaddafi fighters dropped down from the Nafusa mountains in western Libya pushing back Gaddafi fighters from the plains below.

The rebel fighters have managed to reach beyond the town of Bir Ayyad, less than 100 km from the capital Tripoli.

But without the protection of the mountains, and with Gaddafi's forces thought to be arrayed across both flanks on the plains, the new frontline seems hard to defend.

Al Jazeera's Jonah Hull reports from Libya.