On Wednesday, rebel soldiers refused to let civilians past a checkpoint outside the town of Ajdabiya on the road leading to the front, citing strict orders from defected army officers who are leading the opposition forces.
The tightening of the checkpoint seemed to mark another step in the transformation of their uprising from a wave of anti-regime street protests to an armed insurgency in the country's eastern desert.
Until now, the front lines have had a carnival-like quality, with hundreds of young men, mostly unarmed, milling around, singing and chanting, and then fleeing in panic when mortars crash into the sand along the road.
But after a week in which the rebel forces have been pinned down outside a string of vital oil towns they have twice seized and lost, the core of army defectors leading the charge ordered civilians be kept away from the battle.
The move was in part aimed at enforcing greater discipline on the rag-tag army, which mostly consists of ordinary civilians with little to no military training.
"We are all civilians, but the point is to keep out those who have no weapons," says Abdelhadi Omar, 50, a truck driver turned opposition militant. The measures are also aimed at keeping out Muammar Gaddafi's spies, he said.
"Yesterday, everywhere we went, as soon as we stopped, they shelled us. [The rebels] figured out it was a spy and took him back to Benghazi," their eastern stronghold, Omar said.
Ayman Mohammed, a 22-year-old student also fighting on the front, said he saw fellow rebels wrestle the alleged spy to the ground and confiscate his satellite phone and two pistols.
Other rebels describe alternate methods used by spies to give away their position.
"There are people who dress like revolutionaries and act like revolutionaries, but they aren't with us. They work for Gaddafi," says Mohammed Al-Jahri, another rebel fighter. "They go out to the field with us and then they fire their rocket-propelled grenades into the air to give away our location," he said.
Khaled Al-Sayeh, a member of the rebels' military council, says authorities in Benghazi are currently holding around a dozen suspected spies. "The problem has been with us since the start of the revolution, but it isn't a major or widespread phenomenon," he told AFP.
"When we find them we bring them back to Benghazi and interrogate them. If we don't fear that they pose a danger to us we let them go." The rest are held with other prisoners taken during the month-old uprising.
At the checkpoint on the edge of Ajdabiya, the last rebel-held town before the front, some of the young men whose fervor has fueled the uprising chafed against the new restrictions, demanding the soldiers let them go ahead.
But most seemed happy to set up camp out under the nearby western gate of the city, where they made tea over fires, occasionally singing and clapping.
The scene was different some 40 kilometres (25 miles) closer to Brega, where a much smaller group of revolutionaries, nearly all of them armed, milled around at a rest stop they said had been shelled the night before.
"We are the original revolutionaries," said 26-year-old Ahmed Faturi. "We have weapons and we fight and we've been wounded," he added, pointing to a wound on his neck that he said was caused by shrapnel.
He had slept the night before in a flatbed truck loaded with weapons, and had not passed through the checkpoint back in Ajdabiya.
He also claimed to have seen "traitors" fire rocket-propelled grenades in the air to give away rebels to Gaddafi's artillery.
Ten kilometres (six miles) nearer to the front, several dozen armed rebels, mostly bearded men who appeared to have slept outside for several nights, calmly scanned the horizon as artillery thundered in the distance.
Jahri, who just two months ago was a civilian who had never held a gun, paced the road in military fatigues with a Kalashnikov. He has been fighting since the start of the uprising, making him a veteran of sorts.
"I didn't even know how to load this gun in the beginning," he says. "But we learned through all these battles ... Gaddafi has taught us how to fight."