DAMASCUS - The UN human rights chief said on Friday that the killing of 108 civilians by pro-government forces in Syria last week might constitute a "crime against humanity" as reports emerged of a new slaughter.

Syrian activists threatened a "volcano of rage" over the killings near the central town of Houla, in which 49 of the victims were children, as a deadline set by rebel fighters passed for the authorities to respect a UN-backed truce.

Scores of protests erupted around the country in response to the activists' call, as hundreds of mourners took to the streets of the central town of Qusayr for a mass funeral for the 12 workers whom activists said were summarily executed by government troops on their way home from their factory on Thursday.

Activists said security forces opened fire on demonstrators in Douma in the Damascus suburbs, in the main northern city Aleppo, and in Daraa, south of the capital, birthplace of the 15-month uprising against the government of President Bashar al-Assad.

"These acts may amount to crimes against humanity and other international crimes," Pillay told a special session of the Human Rights Council in Geneva, referring to the alleged role of the Syrian military and pro-government militia in the killing near Houla.

The Human Rights Council was meeting to discuss the massacre that triggered global horror and outrage, and debate a call for an independent inquiry into events.

Syria and the entire region are in danger if a full-fledged conflict erupts in the country, Pillay said in a statement read to diplomats.

The High Commissioner for Human Rights called on the international community to throw its weight behind the six-point peace plan brokered by UN-Arab League envoy Kofi Annan.

"Otherwise, the situation in Syria might descend into a full-fledged conflict and the future of the country, as well as the region as a whole could be in grave danger," she said.

Annan's peace blueprint was supposed to begin with a ceasefire from April 12 but the truce has been violated daily with another 13 people killed on Friday after 64 died on Thursday, according to a human rights watchdog.

The 12 workers killed on Thursday were on their way home from a fertiliser factory outside Qusayr, a town south of Houla, also in the flashpoint central province of Homs, activists said.

"The workers were on a bus when they were forced to stop at a checkpoint on the outskirts of Qusayr," said Salim Kabbani of the Local Coordination Committees, which organises protests on the ground.

"Regime forces tied their hands behind their backs and shot them."

Kabbani said abuses had become routine in Qusayr, a town southwest of the flashpoint central city of Homs. "The checkpoint where the workers were killed is dangerous, and people are often tortured there."

Amateur video posted on YouTube by activists showed bodies lain out side by side, several with bullet wounds to the head.

The footage, purportedly filmed when rebel fighters reached the scene to recover the bodies, included the voice of one man crying out: "This is my son, my son," as he tugged in vain at the leg of a corpse lying face up, his blue shirt and white trousers covered in blood.

Another video posted on Friday showed hundreds of people in Qusayr taking to the streets for a joint funeral for the slain workers.

A man bearing the Syrian independence flag led the cortege. Mourners chanted: "Oh God, we only have you to turn to," and: "We won't surrender, we won't surrender."

Green branches were laid on the white shrouds wrapped over the corpses as they were carried by pallbearers through the town.

The Britain-based Syrian Observatory for Human Rights said it had asked the UN mission in Syria to visit Qusayr as soon as possible to investigate the latest killings.

The Obseratory's head Rami Abdel Rahman said that the persistent bloodshed made a mockery of the UN-backed ceasefire that was supposed to take effect from April 12.

"The ceasefire has been dead for a month," he said, adding that a full 2,287 of the more than 13,400 people killed since the uprising began had died since the nominal start of the truce.

With Arab and Western governments at odds with Damascus allies Beijing and Moscow over the best way to tackle the crisis, German Chancellor Angela Merkel held talks in Berlin with Russian President Vladimir Putin.

Putin rejected Western accusations that Russia was siding with Assad's government, insisting it supported neither side in the conflict, but he shared growing concerns of a slide into civil war.

"We are supporting neither side from which the danger of civil war is coming," he told reporters.

He underlined Moscow's position to any talk of Libyan-style military intervention in Syria. "You cannot do anything by force," he said.

Merkel said the two leaders had both agreed on the importance of backing Annan's peace mission.

"We both made clear that we are pushing for a political solution, that the Annan plan can be a starting point but that everything must be done in the United Nations Security Council to implement this plan," she said.