Eight Western governments have expelled Syrian diplomats from their capitals in a coordinated action against Bashar al-Assad's government over the killing of more than 100 civilians in Houla.
French Foreign Minister Laurent Fabius called Mr Assad a murderer and Australia's Bob Carr said those responsible for the massacre would be held to account.
Mr Fabius, however, ruled out ground intervention in Syria, saying the risk the conflict could spread was too great.
Australia, Britain, Canada, France, Germany, Italy, Spain and the Netherlands announced the expulsions after consultations with each other on what they called unacceptable levels of violence.
The action marked a new phase in the international effort to halt the repression of a 14-month-old uprising against Mr Assad and to force him to relinquish power.
The immediate catalyst for the expulsions was Friday's massacre in Houla, including women and children, although the international community is increasingly frustrated at the failure of a UN-brokered peace plan to end the bloodshed in Syria.
Syrian officials denied any army role in the massacre, one of the worst single incidents in the conflict.
British Foreign Minister William Hague said the expulsions aimed to tell President Assad and his ruling elite that time was running out for them to comply with the peace plan.
Some governments told the diplomats to leave immediately, others gave them up to seven days to pack their bags.
Australian Foreign Minister Carr called the Houla killings a hideous and brutal crime.
Turkish Prime Minister Tayyip Erdogan also condemned the killings and said there was a limit to the world's patience.
Some Arab nations have already taken similar diplomatic action against the Syrian government.
Gulf Cooperation Council countries - Saudi Arabia, the United Arab Emirates, Qatar, Bahrain, Oman and Kuwait - expelled Syrian envoys in February then the following month closed their embassies in Damascus.
The new coordinated action from the Western countries took place as international mediator Kofi Annan met Mr Assad in Damascus and told him that "bold steps" were required for his peace plan to succeed.
Mr Annan expressed the international community's grave concern about the violence in Syria, in particular the killings in Houla, his spokesman said.
More than 10,000 people have been killed since the uprising against Assad's 11-year-rule broke out in March 2011, most of them opposition supporters killed by the army or security forces.
Reports from Syria cannot be independently verified as state authorities have barred international journalists and rights groups.
