Syrian government forces have executed 12 civilians on their way home from work in a fertiliser factory in Qusayr, activists in the town have claimed.
The reported killings late yesterday afternoon came on the eve of a special session of the UN Human Rights Council called to discuss the conflict.
Reports from Syria cannot be independently verified as state authorities have barred international journalists and rights groups.
Salim Kabbani of the Local Coordination Committees said abuses had become routine in Qusayr, a town southwest of the flashpoint city of Homs.
Several areas of Qusayr have been under non-stop shelling by government forces, he said.
Amateur video posted on YouTube by activists showed bodies lain out side by side, several with bullet wounds to the head.
Another video posted today 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. Crowds 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 opposition Syrian National Council accused the regime of carrying out the "slaughter" and called for international intervention to stop killings in Syria.
It called on the UN Security Council and the Arab League to take "courageous decisions" against the regime to halt its crackdown and protect the Syrian people.
For its part, the Britain-based Syrian Observatory for Human Rights said it had asked the UN military observer mission in Syria to visit Qusayr as soon as possible to investigate the latest killings.
Observatory head Rami Abdel Rahman told AFP that the persistent bloodshed made a mockery of the UN-backed ceasefire that was supposed to take effect from 12 April.
"The ceasefire has been dead for a month," he said, adding that, according to Observatory figures, a full 2,287 of the more than 13,400 people killed in the 15-month uprising against the government of President Bashar al-Assad had come since the nominal start of the truce.
"In Qusayr, the regime has shelled incessantly in recent days because it is trying to regain control of an area it has lost control of to rebels," Abdel Rahman said.
UN human rights chief Navi Pillay told the council meeting in Geneva that the escalating conflict threatened the entire region.
In a statement read to the session, she called on the international community to throw its weight behind the 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.
Clinton claims Russia still supplying arms to Syria
Meanwhile US Secretary of State Hillary Clinton has claimed that Russia has continued to supply arms to the regime of Syrian President Bashar al-Assad, raising "serious concerns" in the United States.
"We know there has been a very consistent arms trade, even during the past year, coming from Russia to Syria. We also believe the continuous supply of arms from Russia has strengthened the Assad regime," she said at a news conference in Oslo.
"The fact that Russia has continued to sustain this trade in the face of efforts by the international community to impose sanctions ... has raised serious concerns on our part," she said.
Ms Clinton's remarks followed assertions by Russian President Vladimir Putin earlier today that Russia takes no side in the fighting in Syria and was not supplying "the weapons that could be used in a civil conflict."