Beirut ? The U.S. closed its embassy in Syria and Britain recalled its ambassador to Damascus on Monday in a new Western push to get President Bashar Assad to leave power and halt the murderous grind in Syria — now among the deadliest conflicts of the Arab Spring.

Although the diplomatic effort was stymied at the U.N. by vetoes from Russia and China, the moves by the U.S. and Britain were a clear message that Western powers see no point in engaging with Assad and now will seek to bolster Syria’s opposition.

“This is a doomed regime as well as a murdering regime,” British Foreign Secretary William Hague told lawmakers as he recalled his country’s ambassador from Syria. “There is no way it can recover its credibility internationally.”

President Barack Obama said the Syrian leader’s departure is only a matter of time.

“We have been relentless in sending a message that it is time for Assad to go,” Obama said during an interview with NBC. “This is not going to be a matter of if, it’s going to be a matter of when.”

The most serious violence Monday was reported in Homs, where Syrian government forces, using tanks and machine guns, shelled a makeshift medical clinic and residential areas on the third day of a relentless assault, killing a reported 40 people, activists said. More than a dozen others were reported killed elsewhere.

Those deaths followed a regime onslaught in Homs that began Saturday, the same day Syria’s allies in Russia and China vetoed a Western- and Arab-backed resolution aimed at trying to end the crackdown on dissent. Some 200 people died, the highest death toll reported for a single day in the uprising, according to several activist groups.

Even as the U.S. steps up pressure on Assad to halt the violence and relinquish power, Obama said a negotiated solution was possible, without recourse to outside military intervention.

Later, however, White House spokesman Jay Carney said the administration was taking “no options off the table.”