Funeral protest in Rastan neighborhood of Homs in central Syria Feb. 7. According to the U.N., the death toll at the hands of the regime of Syrian President Bashar al-Assad has grown to 5,400 since the rebellion began in March last year. Other estimates put the figure at more than 7,300.

As of Feb. 21 the opposition stronghold of Homs, particularly the city’s shaabi, or working-class districts, had been shelled by government tanks and artillery for 18 straight days.

On Feb. 4 the Russian and Chinese governments both vetoed a U.S.-backed U.N. Security Council resolution condemning the government crackdown. When the rebellion began, Washington offered only mild criticisms of Assad. In spite of sharp differences, the Barack Obama administration had been moving toward improving diplomatic relations, seeing the regime as key to regional “stability,” including maintaining the status quo along its border with Israel.

Washington now calls for Assad’s resignation, seizing on recent events as a way to weaken the growing regional influence of the Iranian government, one of Assad’s closest allies, as it pressures Tehran to abandon nuclear technology.

Over the last several months, some soldiers who have defected from the more than 200,000-strong Syrian army have joined armed resistance to the regime, receiving weapons from undisclosed sources smuggled in from neighboring Lebanon, Turkey, Jordan and Iraq.

Seeking to step up the pressure and further isolate Tehran, Washington is organizing a “Friends of Syria” conference in Tunis, Tunisia, beginning Feb. 24, with close collaboration from the governments of Saudi Arabia, Qatar, the United Arab Emirates and Turkey.

White House press secretary Jay Carney told reporters that “we don’t rule out additional measures” against Syria, widely viewed as an indication Washington is considering openly aiding certain wings of the heterogeneous opposition.

—SETH GALINSKY