BEIRUT (AP) — The U.S. closed its embassy in the Syrian capital of Damascus on Monday in a dramatic escalation of pressure on President Bashar Assad to give up power, just days after diplomatic efforts to end nearly 11 months of bloodshed collapsed at the United Nations.

The U.S. evacuated all its diplomats from the country as Syrian forces intensified a shelling assault on the restive city of Homs. The offensive 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 brutal crackdown on dissent.

The onslaught on Homs has reinforced opposition fears that Assad will unleash even greater violence to crush dissent, now that protection from China and Russia against any U.N.-sanctioned action appears assured.

Already, more than 5,400 people have been killed since the Arab Spring-inspired uprising that began in March, according to the U.N.

“We have been relentless in sending a message that it is time for Assad to go,” President Barack 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 decision to close the embassy is the most dramatic U.S. move so far after 11 months of a violent crackdown by Assad’s regime.