Twelve workers were killed near Syria's western town of al-Qusair when gunmen loyal to President Bashar al-Assad ordered them off a bus and shot them, activists said on Friday, but Syrian media blamed "terrorists" for the killings.
Thursday's killings follow last week's massacre in the Houla region northwest of Homs where the United Nations said 108 people - mainly women and children - were killed in attacks which it said were probably committed by pro-government forces.
The bloodshed in Houla triggered international outrage. Several Western countries expelled Syrian diplomats and demanded tough action against Damascus at the United Nations.
Damascus said a preliminary investigation showed the Houla killings were carried out by gunmen seeking to force foreign military intervention in Syria.
Video released by activists showed bloodied corpses of at least 10 men laid out on the ground near the town of al-Qusair, which like Houla lies about 20km from the opposition stronghold of Homs city. Qusair is southwest of Homs, near the Lebanese border.
Hamza Al-Buweida, a local opposition activist, said he spoke to a survivor who said the dead men had been returning from work at a fertilizer company in al-Buweida al-Sharqiya.
"They stopped, as usual, at a Syrian army checkpoint. But about 300 meters after the checkpoint a yellow car with four armed shabbiha (pro-Assad militia) stopped their car," he told Reuters over Skype.
"They took money off the men and then killed them one by one with gunshots to the head. More than 300 bullets were found in the bodies," he said.
It was impossible to verify Buweida's account of the killing. Syria has restricted journalists' access since the start of the uprising against Assad 15 months ago.
Najati Tayyara, a prominent opposition campaigner from Homs, said Assad's forces were increasingly targeting Sunni civilians as part of a strategy to plunge the country into civil war.
"We are now seeing these massacres increase wherever there are shabbiha present in force. A civil war serves the regime because it mires Syria in a conflict that makes it harder for the international community to intervene," Tayyara told Reuters from Amman.
International mediator Kofi Annan brokered a ceasefire deal seven weeks ago aimed at stemming the bloodshed and finding a peaceful political solution to Syria's bloodshed. But there has been little let-up in the violence, despite the presence of 300 unarmed U.N. monitors in the country.
Activists say 50 to 100 people have died daily this week, including civilians, soldiers and anti-Assad rebels.
On Tuesday, the bodies of 13 men were discovered bound and shot in the east of the country, a killing which the chief U.N. monitor described as an "appalling and inexcusable act". He did not apportion blame but activists said the victims were defectors who were killed by Assad's forces.
The government held prayers to commemorate the dead from Houla on Friday. But anti-Assad demonstrations broke out across Syria after the midday Muslim prayers.
The Islamic militant Al-Nusra Front, which has claimed responsibility for a series of devastating bombings in Damascus and Aleppo which have killed scores of people, called on its followers in Syria to avenge the Houla massacre using force.
"We call the sons of the Front and jihadists in Syria to ignite a fire on the enemy," it said in a statement posted on the internet. "The sons of Islam are waiting for your anger and your response to the Houla massacre as you saw the pictures of the children who were slaughtered with knives and the pictures of women who were killed," it added.