Nearly 850 prisoners have died in Syrian regime jails this year, many executed summarily or tortured to death, the Syrian Observatory for Human Rights said on Wednesday.
"From the beginning of the year until May 13, 847 prisoners, including 15 below the age of 18 and six women, have died in security service prisons and army bases," the Britain-based monitoring group said.
"Families and relatives were notified of the deaths," it added.
"All these people lost their lives as a result of torture, summary executions, maltreatment, poor detention conditions, including a lack of food, and because they were unable to obtain the medicine they needed."
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The Observatory, which relies on a network of activist, medical and military sources in Syria, said the number of deaths in regime prisons was likely much higher because of the difficulty in recording the fatalities.
It said some 18,000 people among those held by the government have disappeared, and many were feared dead.
"The number of victims increases because there are no measures being taken to deter the regime," said Observatory director Rami Abdel Rahman.
"When the criminal knows there is no accountability, he continues his crimes."
More than 150,000 people have been killed in Syria since the conflict there began in March 2011, and nearly half the country's population has been displaced.