* Rebels pull back towards Ajdabiyah
* Biggest retreat for several days
* Rebels blame NATO takeover for weaker air support
(Adds detail, situation at nightfall)
By Alexander Dziadosz
NEAR BREGA, Libya, April 5 (Reuters) - Muammar Gaddafi's forces pushed rebels back towards their stronghold of Benghazi on Tuesday with sustained rocket and mortar fire, in the sixth day of fighting for the oil port of Brega.
Sustained bombardments of rockets and mortars pushed the insurgent caravan of pick-up trucks and cars about half way towards the town of Ajdabiya, gateway to Benghazi. It was the biggest rebel retreat in several days of inconclusive battles, but as night fell the artillery fell silent and there was no sign that Gaddafi's better-armed forces would push on to Ajdabiyah.
The exact position of the frontline was difficult to ascertain in a war of rapid movements in both directions across the open desert. Rebels waiting in their pick-ups about 30 km west of Ajdabiyah on the Mediterranean coast road said there was fighting 10 km west of them, about 40 km east from Brega.
Others said there was still fighting in Brega itself.
The insurgents say Western air strikes have become less effective since NATO took control from a coalition of France, Britain and the United States on March 31.
"Since the day NATO took over the air strikes, we have been falling back," said Ziad el Khafiefy, 20, a rebel fighter.
"Gaddafi's troops are hitting us with (Russian-made) Grad missiles," said Mabrouk el Majbary, 35. "Something is wrong... When the U.S. gave leadership to NATO, the bombings stopped. I don't know why."
NATO denied air power had become less effective.
The retreat began when rockets landed near a group of rebels waiting in pickups with mounted machineguns at Brega's eastern gate on Tuesday morning.
Nearby, the remains of two government trucks mounted with heavy machineguns lay smouldering, their burning tyres giving off a cloud of acrid smoke. Rebels said the trucks were hit by a Western air strike.
The sustained burst of rocket and mortar fire appeared to give the Gaddafi forces the upper hand after days in which lightly-armed rebels pulled back but better-trained soldiers held their ground.
Kamal el-Maghraby, a rebel who returned from Britain to fight, expressed frustration at the superior weapons on Gaddafi's side. "It's not balanced," he said, gesturing towards the rebels' Kalashnikov assault rifles. "These people cannot fight with those weapons."
The rebels had shown better organisation than in past weeks, keeping territory for longer and forcing untrained volunteers to stay back as experienced forces attack Gaddafi's front line. But government troops seemed to mount a sustained assault on Tuesday. (Writing by Barry Moody; Editing by Matthew Jones)