Shocking evidence has emerged of the brutality inside Syrian jails and detention centres.

Key points: Photos were smuggled out by regime photographer codenamed 'Caesar'

Photos were smuggled out by regime photographer codenamed 'Caesar' Human Rights Watch made them available to victim's families

Human Rights Watch made them available to victim's families They document nearly 7,000 individual deaths

They document nearly 7,000 individual deaths Bodies show evidence of torture, beatings, starvation

A new 86-page Human Rights Watch report, titled If the Dead Could Speak, features photographs and testimonies that document the deaths of nearly 7,000 people in detention facilities at the hands of Syria's mukhabarat (security agencies).

Many appear to have died from torture, beatings or starvation. The organisation has demanded the Syrian government grant access for independent monitors to see inside detention centres for themselves.

The photographs were taken by a former forensic photographer for Syria's military police who smuggled them out of the country on discs and thumb-drives and later defected from the Syrian government.

Code-named 'Caesar', it was his job to photograph the bodies of all dead detainees to record the thousands of people who have died in detention since the civil war broke out in 2011, as well as members of the security forces killed by armed opposition groups.

The report says, in total, Caesar smuggled more than 50,000 images out of Syria, including 28,707 that showed the bodies of detainees who died in detention. Many were of the same person, and Human Rights Watch says they correspond to at least 6,786 separate dead individuals, each with their own unique identification number.

The report says Caesar told both the US Congress and an international team of lawyers investigating the photographs that they were all taken at Syrian government military hospitals.

The photographs were eventually handed to the Syrian Association for Missing and Conscience Detainees, which then gave them to Human Rights Watch. They were posted online in March, allowing many Syrian families to finally know what had happened to their loved ones.

'Ahmad was a soul, and he became a number'

Dahi al-Musulmani's 14-year-old nephew Ahmad was among them. He says Ahmad fled to Lebanon in 2012 after his brother Shadi was killed in a protest. But he returned to Syria the following year when his mother died.

Syrian authorities stopped him at the border and found a song critical of President Bashar al-Assad on his mobile phone. Ahmad was taken into custody. His uncle spent nearly two years trying to find him and secure his release.

But when the photographs from Caesar were posted online in March 2015 he found his nephew among them.

"I was surprised to find his photograph," Mr al-Musulmani told Human Rights Watch.

"It was him. His nose, his face ... it was him. It was Ahmad with a number. They put a number on him. Ahmad was a soul, and he became a number."

Human Rights Watch spent nine months finding some of the victims' families to verify the photographs and some of the stories behind them. They sent photographs of 19 victims to forensic pathologists at the organisation Physicians for Human Rights, who confirmed the injuries seen in the photos were consistent with torture, beatings and starvation.

"The pattern of injury was so severe that you could see large blotches of blue discolouration because of the beating they'd sustained," Dr Nizam Peerwani said.

"Some of them showed defensive wounds which implied that the person was aware and cognisant that he or she was being attacked."

Dr Nizam said in the photos he saw the injuries were inflicted and not accidental.

"The location of injuries tell a story. A person stumbling or falling down, usually has injuries on exposed areas of the body," he said.

"He may have scraped the knee or he may have scraped the forehead but you don't find injuries on the abdominal area, in the arms or the inners of the arms and thighs. So the pattern of injuries, there's no doubt they are inflicted injuries.

"The photographs speak for themselves. The patterns of injuries are so clear. They clearly show that these people have been starved to death, many of them. They clearly show injury patterns that are not consistent with falls, they are consistent with torture."

Torture, starvation widespread in regime jails

Former detainees who have since been released, and guards who have also defected, have confirmed the horrendous conditions for those in custody in Syria, and say torture and malnutrition were widespread.

"The [Caesar] photographs show the reality of detention," said one former detainee.

"At first, when I was detained I was shocked by what I saw. Shocked by how emaciated people were, shocked by the dark circles under their eyes. I was shocked. And then, little by little, I became just like them."

Another former detainee said food in his detention centre was scarce.

"In the evening for example, they'd bring us a boiled potato. Sometimes they'd bring us two-and-a-half, or just two potatoes, to share among 12 detainees. You become a skeleton. If a person weighs 90 kilograms or 100 kilograms in detention they go down to 50 kilograms," he said.

"Almost every [detainee] had scabies. I saw detainees who had been in detention for so long their whole bodies, especially their backs, were infected. Many of those detainees died."

Children were also malnourished, a third former detainee said.

"I met a 15-year old detainee who had been there for nine months. He was skin and bones, just skin and bones. No muscle, just bones. You could see his ribcage. He told me, 'Just take care of your hygiene and make sure you don't get any infections'," the detainee said.

Overcrowding too was a serious problem.

"The deadliest thing was the number of [detainees] in the cell," said the first man. "In our cell there were 115 of us."

"For three hours, I would have to stand up," said the second man. "We used to sit down in three hour shifts. After three hours I would lean a little. We would take turns."

Detainee's skin 'would just peel off'

Torture was widespread.

"We went inside and saw a line of detainees against the wall being beaten," said the third man. The guards had whips with two prongs so that with a single swing, the detainees were hit twice. They also beat them with cables and thick, heavy ropes.

"There was a detainee from Damascus," said the second man. "He'd be called up twice a day for interrogation and he wouldn't give them anything. After a week of interrogating him he came back drenched in blood. His skin, if you just touched him, it would peel off. It was horrific torture."

The Human Rights Watch report says that torture, beatings and even mild infections were fatal on an almost daily basis.

"There were rarely any days when no prisoner died," a former guard at one detention facility told HRW.

"Most days, there were three, four, five, six deaths, and sometimes up to 11 a day. One day I saw 11 prisoners dead."

Human Rights Watch said the bodies of dead detainees were transferred to one of two military hospitals in the Damascus area. They were photographed initially in morgues, and later in a courtyard that Caesar identified as being a garage of one of the military hospitals.

In 2012 the organisation identified and mapped 27 detention centres around the country, including many in Damascus. Satellite images and geolocation techniques, as well as testimony from defectors from the two military hospitals, helped establish that many of the photographs were taken at the Syrian military hospital known as 601 at Mezze, in Damascus.

"We used to use the morgue," Caesar told the US Congress.

"But they were bringing way more bodies, so we decided to start using the garage."

Human Rights Watch Deputy Director for the Middle East, Nadim Houry, said the Syrian Government must allow independent monitors in to see conditions in detention centres for themselves.

"What is needed today is not just some releases. What is needed is finally a real push from the internationals for independent monitors to be finally granted access to detention facilities," he said.

The Syrian Government has previously and consistently rejected photographs or any evidence of torture inside its prisons.

In an interview with Foreign Affairs magazine earlier this year, Mr Assad said: "You can bring photographs from anyone and say this is torture. There is no verification of any of this evidence, so it's all allegations without evidence."