Major Western powers say they are expelling senior Syrian diplomats following the killing of 108 people in the Houla region of Syria on Friday.
The United States, France, the UK, Germany, Italy, Spain, Canada, Australia, the Netherlands and Switzerland all took action.
Most of the victims in Houla were summarily executed, the UN says.
UN-Arab League envoy Kofi Annan said, after talks with President Bashar al-Assad, Syria was at a "tipping point".
Mr Annan said he had asked Mr Assad to take "bold steps" to see that his peace plan was implemented.
President Assad blamed the violence on "terrorists". His remarks were quoted by state TV.
Earlier, UN human rights spokesman Rupert Colville said initial investigations had suggested that most of those killed in the village of Taldou, near Houla, were summarily executed.
He said 49 children and 34 women were among the victims. UN observers who visited Taldou said many of the victims had been killed by close-range gunfire or knife attacks.
Eyewitnesses told the BBC that pro-government shabiha militiamen had carried out the killings. Survivors said they had hidden or played dead.
Syrian leaders insist that the massacre was the work of "terrorists", aiming to derail the peace process and provoke intervention by Western powers.
Violence continued on Tuesday, with nearly 50 people killed in various incidents, according to activists.
Read more at the BBC....