* Slow progress is testing NATO unity
* Heavy casualties for little ground gained
* Hazy command structure leading to tactical blunders
By Matt Robinson
MISRATA, Libya, June 19 (Reuters) - The fighting in farmland west of the port city of Misrata is exposing the weakness and inexperience of rebels who are trying to advance the Libyan capital Tripoli.
The rebels are taking heavy casualties for little ground gained. And the slow slog is also testing the unity of the NATO alliance which supports them from the air.
Eight more rebels had died by early Sunday afternoon in heavy artillery barrages, bringing the toll in just over a week to more than 40.
About 200 have been wounded -- a huge loss in manpower for a rebel force that has already expended so much to break out of Misrata, scene of some of the bloodiest fighting in Libya's four-month-old uprising.
Tactical missteps appear to be rife as rebels run into the heavy guns of forces loyal to Libyan leader Muammar Gaddafi on the outskirts of Zlitan, the immediate target of fighters trying to push 200 km (125 miles) from Misrata to Tripoli.
As ambulances and pick-ups rushed the wounded to a field hospital near Misrata's western front on Sunday, senior rebel Mohammed Ali pulled in from the front.
"We made a mistake today," he said. "We sent the boys out on foot before the vehicles."
Two days earlier, he railed against other units who, he said, had opened fire too early and alerted pro-Gaddafi forces to an offensive through olive groves some 10 km short of Zlitan.
"We had a strategy to finish everything today, but some of the fighters think it's a game," he said on Friday.
On three fronts of the Libyan war, the rebel advance has slowed considerably, sometimes to a near-standstill.
Now entering its fourth month, NATO's air campaign over Libya has dragged on for longer than its 11-week war in 1999 to drive Serb forces under Slobodan Milosevic out of majority-Albanian Kosovo.
Milosevic stayed in power for more than a year after the bombing stopped.
There are growing fissures in the 27-member Western military alliance, which was already uneasy when the war began. Some have voiced doubts over their ability to sustain the campaign.
But the rebels appear incapable of providing a knock-out blow.
The Misrata rebels honed their fighting skills in close-quarter street battles, wresting the city centre from pro-Gaddafi forces and then pushing them back on three fronts to break an artillery siege.
They are proving less successful in open ground.
Trying to clear a path to Zlitan, the rebels have resorted over the past week to heavy artillery.
On Friday, a rebel manning a Russian-made multiple rocket launcher said they were aiming at tanks and munitions.
Asked what experience they had firing Grad rockets, he replied: "We were trained for seven days."
On this strip of the Mediterranean coast, the vast majority of rebel casualties are caused by shrapnel from mortars and artillery.
One strike often causes multiple casualties given that the rebels frequently gather together, either when advancing through fields or eating lunch in the defensive lines.
On Friday, men of the Swehli unit - led by descendants of Ramadan al-Swehli, lionised in Misrata for his fight against the Italian occupation - stationed themselves at a battered mosque in the Naimah region outside Zlitan.
The minaret was visible for miles across flat farmland, a perfect target for pro-Gaddafi forces who landed artillery rounds in a tightening arc just a couple of hundred metres from the mosque. The rebels barely flinched.
After hours of artillery exchanges, there was no discernible change in positions but six rebels left the fight with severe shrapnel wounds.
On Sunday, the unit was ambushed as it set out on foot. A fighter named Mustafa died, and was brought to the hospital on the back of a pick-up truck under a blanket.
The Swehli unit is built on family ties and friendship. "In our group, we're all close friends," said Mohammad Swehli.
Seniority stems more from age or tribal prestige than military know-how, leaving a command structure that is hazy at best and prone to collapse.
At the very front, it's the same faces day in, day out. They are determined but few in number. Many more man checkpoints or lay on rugs in ditches to hold the back lines.
"Our units, we're the only ones fighting," Ali said. "The rest of Misrata is sleeping or swimming in the sea."