U.S. President Barack Obama on Saturday decried the "relentless brutality" of the regime of Syrian President Bashar al-Assad and urged the United Nations Security Council to take a stand.
Mr.Obama called the reported killing of 200 people in Homs by government shelling an "unspeakable assault," and urged Assad to step down.
Security Council ambassadors gathered Saturday morning to consider a draft resolution backing an Arab League call for Assad to cede power.
The unusual weekend session comes as Syrian forces pummel the city of Homs with mortars and artillery in what activists are calling one of the bloodiest episodes of the uprising.
Although Western nations are pressing for a vote, there's no guarantee the resolution will pass.
At a security conference in Munich, Russian Foreign Minister Sergey Lavrov said that Moscow still sees two problems of "crucial importance" with the draft resolution on the violence in Syria, but that consensus is still possible.
Western countries have been working to head off a possible Russian veto if the resolution goes to a vote.
Meanwhile, Tunisia - the birthplace of the "Arab Spring" uprising - has decided to expel Syria's ambassador in response to what it calls a "bloody massacre" in Homs. A source in the presidential office says Tunisia no longer recognizes the regime of Syrian President Bashar Assad.
Demonstrators have stormed five Syrian embassies in Europe and the Middle East. Protesters forced their way into the Syrian embassies in London and Athens, prompting some arrests and causing minor damage. Similar disruptions were caused in Berlin and Cairo.