Syria is committing crimes against humanity as part of state policy to exact revenge against communities suspected of supporting rebels, Amnesty International said in a report.
The London-based rights group called for an international response after claiming it had fresh evidence that victims, including children, had been dragged from their homes and shot dead by soldiers, who in some cases then set the remains on fire.
"This disturbing new evidence of an organized pattern of grave abuses highlights the pressing need for decisive international action," said Amnesty's Donatella Rovera on release of the 70-page report entitled Deadly Reprisals.
The charity interviewed people in 23 towns and villages across Syria and concluded that Syrian government forces and militias were guilty of "grave human rights violations and serious violations of international humanitarian law amounting to crimes against humanity and war crimes."
Reporting on the revolt which broke out in March last year, Amnesty described how soldiers and shabiha militias burned down homes and properties and fired indiscriminately into residential areas, killing and injuring civilian bystanders.
"Everywhere I went, I met distraught residents who asked why the world is standing by and doing nothing," said Rovera.
The report also accused the regime of routinely torturing those who were arrested, including the sick and elderly.
In the report, Amnesty called on the United Nations Security Council to refer the case to the Prosecutor of the International Criminal Court (ICC), and to impose an arms embargo on Syria.
According to the Britain-based Syrian Observatory for Human Rights, more than 12,000 people have been killed since the uprising against President Bashar al-Assad began, most of them civilians.
Regime troops and rebels clashed again across the country on Wednesday, the watchdog said, as opposition fighters withdrew from the besieged town of Al-Haffe after eight days of intense shelling.
At least 34 people were killed in violence, among them 27 civilians, six soldiers and one rebel, according to the Syrian Observatory for Human Rights.
The escalation in violence follows an assessment by UN peacekeeping chief Herve Ladsous on Tuesday that Syria was now in civil war, with regime forces having lost control of "large chunks of territory."