The shooting near the town of Qusair in Homs province happened as the 12 fertiliser workers travelled in a bus that came under fire, said the Britain-based Syrian Observatory for Human Rights.
A pro-government Facebook page, the Homs News Network, posted photos of the men on the floor of what appeared to be a classroom. It blamed the rebel Free Syrian Army, saying the workers were killed for being state employees.
The opposition blames the government and said the killings were carried out by the shabiha, a notoriously brutal armed militia loyal to Syrian president Bashar Assad.
Thursday’s shootings followed a day after 13 bound corpses, many apparently shot execution-style, were found in the eastern province of Deir el-Zour near the Iraqi border. The men were believed to be workers for an oil company.
Homs province, where there is significant support for the opposition, has suffered waves of deadly violence during the uprising which is believed to have claimed over 9,000 lives.
More than 100 people were killed there during a massacre in a cluster of villages known as Houla on Friday and Saturday last week. Many of the dead were women and children.
It brought immediate worldwide condemnation and regime and anti-government activists have blamed each other, with opposition groups calling for international action. They say that the ceasefire brokered by former UN chief Kofi Annan as part of a staged peace plan lies in tatters.
Syria on Thursday claimed up to 800 rebel fighters carried out the Houla massacre, but the government’s narrative has been starkly contradicted by eye witnesses who blamed the shabiha.
The UN also said it had strong suspicions the pro-regime gunmen were responsible for much of the carnage.
The UN’s top human rights body voted yesterday to condemn Syria over the killings. The 47-nation body approved with 41 votes against three a resolution blaming “pro-regime elements” and government troops for the Houla massacre. Russia, China and Cuba voted against, while two countries abstained and one did not vote.
The resolution also call for an “international, transparent, independent and prompt investigation” and echoed calls by UN rights chief Navi Pillay for the UN Security Council to consider referring Syria to the International Criminal Court.
There has been growing anger at Russia’s and China’s stance.
Yesterday Russian President Vladimir Putin said his country wants to help UN envoy Mr Annan achieve “positive results” and prevent an all-out civil war.
He said Russia will remain in contact with Mr Assad and the Syrian leadership, insisting nothing could be imposed by force, but was vague about what “political instruments” Russia might be prepared to use.
US Secretary of State Hillary Clinton dismissed his claims of impartiality, however, and claimed the Russia’s stance and military supplies were actually propelling Syria towards civil war.
British Foreign Secretary William Hague urged Syrian opposition groups to build a united coalition against the “murderous” Assad regime. Speaking from Istanbul, where he also met UN secretary general Ban Ki-moon, he said he had sought agreement on how the international community could help forces opposed to Assad.
Earlier, amid reports of the latest mass killing and increasing pressure for outside intervention, Mr Hague said a military response could not be ruled out but would need international support.