BEIRUT - Syrian rebels using improvised mortar bombs made of cooking gas canisters killed 311 civilians between July and December this year, a monitoring group said on Friday, condemning the use of the wildly inaccurate weapons.

Two-thirds of the deaths, or 203 people, were in the northern city of Aleppo where the so-called “hell cannons” have been fired on government-held districts of Syria’s second city.

The Britain-based Syrian Observatory for Human Rights, which tracks the violence using sources on both sides, said that 42 children and 25 women were among the dead in Aleppo. It said more than 700 people had also been wounded during that time.

Syria’s official news agency SANA said on Thursday that “terrorists” fired 11 of the improvised bombs in the southern city of Deraa, wounding several civilians.

The canisters are packed with explosives, fitted with a guide fin and fired by large cannons. Syria’s war started with a pro-democracy movement that grew into an armed uprising and has inflamed regional confrontations. Some 200,000 people have died, the United Nations says.

Chemical weapons have been used, the international chemical weapon watchdog says, and the United Nations says that President Bashar al-Assad’s forces have dropped improvised and indiscriminate barrel bombs on Aleppo.

Meanwhile, Denmark faces a “significant” threat from radicalised Muslim citizens returning home from Syria and Iraq where at least 110 people have gone to fight with militant groups like Islamic State, the Danish intelligence service (PET) said on Friday.

It said at least 16 of those who had gone to Syria and Iraq from Denmark had been killed in fighting and that about half of the more than 100 who had travelled there had since come home.

Denmark is among a range of Western European countries struggling to stop the radicalisation of young Muslim citizens and deter them from becoming militants in Syria or Iraq, fearing they might return to plot attacks on home soil.

In addition, Denmark has sent seven F-16 fighter jets to Iraq as part of the U.S. coalition now conducting air strikes on Islamic State, raising fear of reprisals at home.

“(The agency) assesses that the number of individuals travelling from Denmark to the conflict zone totals at least 110, but that the number may be higher,” the PET said, adding that a small number of women had travelled there too.

None of those returning have been prosecuted because travelling to Syria and Iraq is not illegal but they are being monitored by the PET, a security source said. Some of those back have travelled to the regions many times, PET’s report said.

With a population of 5.7 million, the number of Danish citizens travelling to join hardline Islamist groups in Syria and Iraq is one of the highest per capita in Europe. Thousands of people with Western passports have been fighting there. Muslims account for up to 7 percent of the population although estimates vary.

Meanwhile, activist Mohsen al-Masri spent two years being dragged between prisons controlled by the Syrian security services, enduring savage beatings and being hanged from the ceiling for hours at a time.

But one of the worst horrors he recalls came when his guards started spraying insect repellent around the cell.

“Cockroaches started coming out of everywhere,” Masri says. “The cockroaches started to walk on our faces. The wardens put them inside our clothes.”

Masri is one of thousands of former detainees in Syria’s sprawling prison underworld. He describes a litany of torture that has been not only barbaric but also systematic.

Survivors and lawyers say there are now more than 100 detention centres holding around 200,000 people jailed since the revolt against President Bashar al-Assad began in 2011.

According to the Syrian Observatory for Human Rights, up to 12,000 people have died in these prisons.

Mohammad Samaan, a 33-year-old activist from Damascus, remembers reading George Orwell’s “1984”, the novelist’s dystopian vision of life under an all-knowing dictatorship, in the days before the uprising. “When I became a detainee myself, I discovered that there is such a world, and it is in Syria,” he says.

“Nothing, no amount of reading or listening to other people’s stories could have prepared me for the horror of detention,” adds Samaan, who now lives in Beirut.

Speaking quietly as he pulls on a cigarette, Samaan says he was jailed twice for activism against Assad’s regime, enduring physical and psychological torture on both occasions.

An interrogator at one of Damascus’s feared security branches told him: “We torture people because we are sadists. We enjoy torturing people.”

“He electrocuted me and told me to write down everything I knew. He tried all he could to break me. I have never been so terrified in all my life,” the brown-haired Samaan says.

Masri, 36, says he also endured psychological torture.

“They would insult my wife, and they would tell me they would go to the house and rape her,” he recalls.

At the scores of security offices where detainees are usually first held for questioning, food, water and medication shortages are particularly extreme.

Round-faced Masri weighed more than 100 kilos (220 pounds) when he was first detained. By the time he was released he had lost more than half his body weight.

Like most detainees, Samaan and Masri - who spoke to AFP on condition their names were changed - were transferred from secret detention in Damascus to the infamous Adra and Saydnaya jails after trials they dismiss as farcical.

Masri’s case was heard before a military court despite his non-violent activism.

“The regime doesn’t respect its own laws when it comes to the detainees,” says one Syrian human rights lawyer, speaking on condition of anonymity for fear of repercussions.

“There are four security agencies in Syria, and each does all it can to prove it is more brutal than the next.”

The lawyer described a hellish network of jails, security offices and secret detention centres across the country.

“In Damascus alone there are 30 to 40 security offices, which are illegal, as well as an unknown number of secret detention sites,” he says.

In June, Assad issued an unprecedented amnesty covering tens of thousands of people detained throughout the conflict under Syria’s notorious anti-terrorism law.

But only a handful of high-profile prisoners of conscience have been freed.

Human rights activist Sema Nassar says the regime refuses to release peaceful activists who played a key role in the 2011 uprising because “it fears the impact” they might have if freed.

Syria’s conflict began as a peaceful movement demanding democratic change, but evolved into a civil war after the regime unleashed a violent crackdown against dissent.

Activists say most of those who led the uprising are now dead, in jail, or missing.

Those who did survive detention say they can never forget what they endured.

“Memories come back to haunt me every day, when I eat, when I sleep,” says Samaan. “It’s so awful in there. Some things you just can’t talk about.”