WASHINGTON - Western powers ordered Tuesday the expulsion of Syrian diplomats as they blamed the Houla massacre squarely on President Bashar al-Assad's regime and warned him that time was running out.

The joint action followed mounting international outrage over the massacre on Friday in the central town of Houla, in which at least 108 people, including 49 children, were killed, according to UN figures.

Australia, Britain, Canada, France, Germany, Italy, Spain and the United States acted in concert to step up the pressure on Assad's regime after the killings, many of them summary executions blamed on pro-government militia. "We hold the Syrian government responsible for this slaughter of innocent lives," said US State Department spokeswoman Victoria Nuland, telling Syrian charge d'affaires Zuheir Jabbour that he had 72 hours to leave the country. In her written statement, Nuland said the massacre was "the most unambiguous indictment to date" of Syria's flagrant UN Security Council violations and urged all countries to "condemn the actions of the Assad regime through similar action."

Bulgaria also announced it was expelling Syria's interim ambassador and two other diplomats to protest the brutal killings. British Foreign Secretary William Hague said the expulsion of the top Syrian diplomat in London, charge d'affaires Ghassan Dalla, and two other envoys would send a "stark message" to Assad.

The move was aimed at increasing pressure on senior figures in the regime, "to get the message across to them that they have to choose, that time will run out for Assad," Hague said. Dalla was given seven days to leave Britain. Syria had already withdrawn its ambassador from London. Canada said it was expelling every Syrian diplomat in the capital. "Today, Canada is expelling all Syrian diplomats remaining in Ottawa. They and their families have five days to leave Canada," Foreign Affairs Minister John Baird said in a statement.

German Foreign Minister Guido Westerwelle said: "We are aiming to ensure that our unmistakable message does not fall on deaf ears in Damascus."

In Paris, President Francois Hollande told journalists that France's decision to expel Ambassador Lamia Shakkur, which would be formally communicated to her on Tuesday or Wednesday, came amid talks with Britain, Russia and the UN on the next steps to take in the Syria crisis.

Fabius, France's new foreign minister, forcefully condemned Assad and called for his departure in an interview published Tuesday.

Meanwhile, UN-Arab League peace envoy Kofi Annan urged Syrian President Bashar al-Assad to "act now" to end 15 months of bloodshed, during a meeting in Damascus on Tuesday.

"I appealed to him for bold steps now - not tomorrow, now - to create momentum for the implementation of the plan," Annan told reporters in the Syrian capital.

Annan arrived in the country on Monday for talks with Assad, the opposition and representatives of Syrian civil society.

In his meeting with Assad, Annan conveyed "the grave concern of the international community about the violence in Syria, including the recent shocking events in Houla."

"We are at a tipping point," said Annan, who has previously warned that Syria might slide into civil war should abuses continue. "The Syrian people do not want the future to be one of bloodshed and division. Yet the killings continue and the abuses are still with us today," he added.