The UN Security Council must refer war crimes committed by both sides in Syria's two-year conflict to the International Criminal Court, Amnesty International said on Thursday.
"How many more civilians must die before the UN Security Council refers the situation to the prosecutor of the International Criminal Court so that there can be accountability for these horrendous crimes?" asked Ann Harrison, Amnesty's deputy director for the Middle East and North Africa.
The London-based human rights watchdog has repeatedly accused both the regime of President Bashar al-Assad and opposition fighters in Syria of war crimes and other abuses.
"While the vast majority of war crimes and other gross violations continue to be committed by government forces, our research also points to an escalation in abuses by armed opposition groups," Harrison said in a statement.
"If left unaddressed such practices risk becoming more and more entrenched -- it is imperative that all those concerned know they will be held accountable for their actions."
Amnesty said it has documented regime forces' use of "internationally banned weapons against civilians", and "the torture and summary killing of soldiers, pro-government militias and civilians" captured or abducted by rebel fighters.
It had also researched the army's use of ballistic missiles on the northern city and province of Aleppo.
"Hundreds of residents, many of them children, were killed and injured in three such recent attacks which wiped out entire families," said the rights group.
It gave the testimony of one woman, identified as 31-year-old Sabah, who lost three daughters, her husband, her mother, her sister and her other sister's three sons in one missile attack.
"They were all killed; what is left for me in this life?" Sabah is quoted as saying.
Also in Aleppo, "the bodies of men and boys -- shot in the head, hands tied behind their backs -- are recovered almost daily from the river," Amnesty said.
"The bodies float downstream from a part of the city under the control of government forces."
The organisation also documented abuses by rebel fighters, among them the use of child fighters and even an executioner.
"A video... shows a boy apparently aged between 12 and 14 holding a machete standing over a man -- later identified as Colonel Izz al-Din Badr," said Amnesty.
"He lies prostrate on the ground with his hands behind his back. A voice in the background shouts: 'He doesn't have the strength.' The boy brings the machete down on the man's neck, cheered on by members of an armed opposition group."
Tens of thousands of people have been killed and hundreds of thousands uprooted in the conflict, which erupted in March 2011 with Arab Spring-inspired protests that led to an armed insurgency following a brutal crackdown.