Shot in the back as they fled police, the killing of three unarmed students in Thailand's insurgency-hit south has inflamed controversy over a culture of impunity among security forces that activists say boosts support for rebels.

Most of the 5,700 people who have died since the conflict erupted in the Muslim-dominated region in 2004 have been civilians caught in rebel attacks and assassinations, or raids by police or military forces.

Students Lookman and Samree Dueramae -- brothers aged 20 and 17 -- and their childhood friend Ismael Paetoh, 17, were shot dead in a police-led raid on their hamlet in Krong Pinang district, Yala province, in April last year.

Two suspected Muslim militants, the apparent targets of the raid, were also killed when police and soldiers found the group taking shelter from rain in a lean-to in a rubber plantation.

"My sons were not rebels, they were good boys... but they were shot like they were cats or chickens," said Mariyea Bersa, mother of Lookman and Samree.

From her wood-beamed house overlooking the plantation, Mariyea recalls her sons leaving home to catch birds in the nearby forest.

"I heard gunshots and a while later I saw an army ranger pass my house with a bird cage in his hand," she said, her sons' frayed mattresses still leaning against a wall behind her. "Then I knew my sons were dead."

Villagers wearily accept they are powerless to hold to account the militants, who target Thai forces, or people perceived to be working for the authorities but they demand justice when civilians are killed by police and soldiers.

The kingdom's security forces have faced allegations of abuses ranging from arbitrary detentions and torture to killings of civilians.

Yet none has been successfully prosecuted in the near-decade of fighting.

An inquest in August showed that although Lookman, Samree and Ismael were not suspects they were still shot several times -- including in the back -- by policemen and soldiers who "could not separate who was who".

Absolving the security forces of blame, the ruling said the authorities "were acting in the line of duty".

That justification holds little weight with the victims' families, while rights groups say such cases make good recruiting tools for the insurgents.

Killings by authorities strip villagers of "confidence in the judicial process" according to lawyer Adilan Ali-ishak of the Muslim Attorney Centre, whose organisation is representing the families.

"It's a loss for the government as villagers won't tip them off or act as witnesses because they don't feel the state offers them fairness or protection."

A recent report by the kingdom's Office of the Attorney General (OAG) criticised authorities for detaining suspects for long periods without trial, while citing examples of forced confessions, torture and accidental killings of civilians.

Locals still recall with fury the deaths of 85 anti-government protesters in 2004 at Tak Bai, the majority of them by suffocation as they were stacked -- hands bound -- on top of each other in army trucks.

That issue became a clarion call for rebels while the cascade of allegations of abuses since has also undercut Thailand's efforts to win "hearts and minds" in the deep south --- which was annexed by the kingdom a century ago.

The head of Thailand's army in the south told AFP there have historically been cases of soldiers torturing suspects or killing civilians.

"There are no such cases now," said Lieutenant General Sakol Chuentrakool, commander of the Fourth Army.

"Soldiers do not intend to kill civilians. No one does it on purpose... their job is to protect the people."

But he conceded "old wounds" are used by the rebels "to accuse us, and pull the people in" to their movement, which is fighting for a level of autonomy from Bangkok.

While the authorities say they are making efforts to curb civilian deaths, rights groups note a disturbing rise in the number of alleged extra-judicial killings of rebel suspects on government blacklists.

Such killings go unpunished in a murky and often highly localised conflict, making it impossible to verify who pulled the trigger.

But suspicion frequently falls on shadowy pro-government forces settling scores or taking out rebel commanders.

"There are people out there who think they have to take their own form of justice," said Hadee Hamidong, of peace group the Asia Foundation, which helped with the OAG study.

Several rounds of tentative peace talks have been held between Thailand and some rebel groups but, with a breakthrough seemingly distant, war-weary villagers remain caught between the two sides.

His hand bent by a stroke, Ishak Bersa, the 80-year-old grandfather of Lookman and Samree, said the house was quiet without the two brothers.

"I close my eyes to remember their faces, their figures running around the house," he said.

"We are angry that they were killed... but there is nothing we can do."