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State Department spokeswoman Victoria Nuland said the bombardment marked a “significant and alarming escalation” of the conflict.

Meanwhile, the rebels have made significant advances on the ground over the past couple of weeks as they have overrun air bases and military installations in and around the northwestern city of Aleppo and in the capital, Damascus.

Assad is losing his grip,” a Turkish official said on background. “We see an arc controlled by the opposition, from the east of Damascus to the southwest of Damascus.”

The effects of the civil war are likely to be felt in Syria and the region long after the Assad regime falls, analysts said.

“The opposition has at best limited unity, and elements of the regime might fight on even if Assad goes,” said Daniel Byman, deputy director of the Saban Center for Middle East Policy at the Brookings Institution.

But Marina Ottaway, a Middle East analyst at the Carnegie Endowment for International Peace, warned: “There is a real danger that the international community is backing a [rebel group] that does not have much acceptance inside Syria.”