Opposition activists in Syria say there has been another mass killing of civilians by a pro-government militia.
The claim comes ahead of an emergency meeting of the United Nations Human Rights Council on last week's massacre of 108 people in the town of Houla.
The killings on 25 May triggered worldwide condemnation and led many Western powers to expel Syrian diplomats.
UN observers said some of the victims were killed by shell fire but most appeared to have been shot or stabbed at close range. The dead included 49 children and 34 women.
The UN's Human Rights Council - the world's leading human rights body - is due to hold an emergency meeting on Syria on Friday at which it is expected to condemn the violence in Houla.
The BBC reports more horrendous images from video have been posted on the internet by activists documenting what they say is yet another mass killing by regime forces.
One video showed a group of bodies sprawled on the ground with injuries consistent with their having been shot dead at close range.
Activists said the 12 men killed on Thursday were workers from a fertiliser factory whose bus was intercepted in a village near Qusayr in the west by regime militia known as the shabiha.
According to the account the shabiha they first robbed the workers, then took them off the bus and shot them dead.
UN warns of civil war
United Nations Secretary-General Ban Ki-moon has warned that massacres of civilians of the sort seen in Houla could plunge Syria into a civil war from which the country would never recover.
Mr Ban was speaking at a conference in Istanbul on Thursday, citing fears raised by Arab League-UN special envoy Kofi Annan earlier this week that Syria may have already reached a "tipping point".
A ceasefire on 12 April that forms part of a six-point plan by Mr Annan to restore peace has so far failed to hold, Reuters reports.
US Secretary of State Hillary Clinton also warned Russia that its policy on Syria is contributing to a civil war.
Mrs Clinton said in Denmark that every day of slaughter in Syria was strengthening the case for tougher international action.
However, she stressed military intervention needed international support, especially from Russia and China. Both countries have renewed their opposition to tougher UN Security Council action.
The American ambassador to the UN, Susan Rice, says a reported Russian arms shipment to Syria is reprehensible, although it does not break any laws. She called the results of a Syrian government inquiry into the massacre in Houla a blatant lie.
Damascus has said anti-government armed groups were responsible.
British Foreign Secretary William Hague says the European Union is drafting new sanctions against Syria and wants other nations to do the same. He says pressure must be increased on President Bashar al-Assad to comply with an international peace plan.
Resignation of honorary consul
An honorary consul for Syria in the United States resigned on Wednesday.
Hazem Chehabi, Syria's Consul General in California, said silence or inaction had become ethically and morally unacceptable.
He said the massacre in Houla was a point beyond which he could not justify remaining silent.