GENEVA — The number of deaths in Syrian prisons rose sharply in 2011, Amnesty International said Tuesday, describing it as one of the "most shocking features" of the regime's crackdown on demonstrations.

"No less than 88 such deaths have been reported to Amnesty International as occurring during the period from April 1 and August 15," including 10 children aged between 13 and 18, the human rights group said.

"The sharp rise in the number of reported deaths in custody has been one of the most shocking features of the government's bloody crackdown on the protests," the London-based group added.

This figure was "already many times higher than the yearly average over recent years," Amnesty said, noting the previous average annual death rate was five.

All 88 fatalities were male.

For at least 52 of them, Amnesty said "there is evidence that torture caused or contributed to the deaths."

"Such an increase in deaths cannot be a coincidence. It appears to be the expression of the same brutal violence that is being shown daily in the Syrian streets," said Reto Rufer, who heads the Middle East for the Swiss chapter of Amnesty International.

The rights group had examined 45 videos filmed by relatives or human rights defenders, after their bodies were returned to families. Twenty of the videos were also shown to forensic experts.

The corpses showed signs of violent beatings, burn marks and cuts, noted the group.

"Some of the dead ... were also mutilated either before or after death in particularly grotesque ways apparently intended to strike terror into the families to whom their corpses were returned," said Amnesty International.

The rights campaigner launched an urgent appeal to the UN Security Council to to condemn the mass violations and take measures to bring those responsible to account.

"The reaction of the UN Security Council has so far been inadequate," said the rights group, pointing to China, Russia, Brazil and India's opposition to any stronger response.