French Foreign Minister Laurent Fabius called Assad a murderer and Australia's Bob Carr said those responsible for the massacre at Houla would be held to account.
"Bashar al-Assad is the murderer of his people. He must relinquish power. The sooner the better," Fabius said in an interview with French daily Le Monde.
After diplomatic consultations, Australia, Britain, Canada, France, Germany, Italy, Spain and the United States announced the expulsions of Syrian highest diplomat in their country on what they called unacceptable levels of violence.
The Netherlands declared Syria's ambassador - who lives in Brussels and also represents Damascus in Belgium - to be persona non grata.
However, a EU-wide expulsion has not been decided. Reportedly, the Syrian ambassador to Austria would not be expelled as he holds an additional function as the representative to the UN organizations in Vienna. The same was decided for the Syrian Ambassador to Belgium, who also is representing his country to the European Union.
The action marked a new phase in the international effort to halt the repression of a 14-month-old uprising against Assad and to force him to relinquish power.
But such moves are largely symbolic. In New York, where the United States and its European allies have tried in vain since last year to persuade Russia and China to back sanctions against Damascus, the 15-nation Security Council remains deeply split.
The Russian Ambassador to the EU Vladimir Chizhov criticised the Western expulsions of Syrian envoys: “I believe it rarely is conducive to maintaining dialogue with the other side,” he told the Brussels press yesterday (29 May).
The immediate catalyst was Friday's massacre in Houla, including women and children, although the international community is increasingly frustrated at the failure of a U.N.-brokered peace plan to end the bloodshed in Syria.
"This is a country that is committing such horrors that we cannot deal with them at an ambassador level," a French diplomatic source said. "This decision was made following the Houla massacre. It was coordinated among several countries."
In Washington, the State Department said the Houla had been targeted for "a vicious assault involving tanks and artillery - weapons that only the regime possesses."
"We hold the Syrian government responsible for this slaughter of innocent lives," State Department spokeswoman Victoria Nuland said in a statement, encouraging other countries to express their condemnation by expelling Syrian diplomats.
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 Assad and his ruling elite that time was running out for them to comply with the peace plan.
"The world, the international community, is appalled by the violence that has continued, by the behavior of the regime, by the murder of so many innocent people, including in the terrible massacre at Houla," Hague said.
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. "The Syrian chargé d'affaires has again been advised to convey a clear message to Damascus that Australians are appalled by this massacre and we will pursue a unified international response to hold those responsible to account," he said.
Turkish Prime Minister Recep Tayyip Erdoğan also condemned the killings and said there was a limit to the world's patience.
France's Fabius, however, ruled out ground intervention in Syria, saying the risk the conflict could spread was too great.
"The Syrian army is powerful. No state is ready to consider ground intervention at the current time," Fabius said.
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 Assad in Damascus and told him that "bold steps" were required for his peace plan to succeed.
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.
Assad has been able to count on the support of Russia and China, who have vetoed two moves in the U.N. Security Council to condemn Assad. Russia said on Monday it was alarmed by the killings in Houla but that it believed both sides were to blame.