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In the Syrian areas they control, said Mr. Zelin, al-Nusra had banned alcohol, enforced the veil and introduced lashing as a punishment for fraternization between men and women. But the group so far appeared to be avoiding the more extreme manifestations of their austere version of islamic law, like public beheadings and stonings, or banning music or women leaving their houses without their husband or a male relative.

Mr. Zelin said a backlash against al-Nusra’s social agenda “could be a long or medium terms problem, but is not an issue in the short term.”

The news from Syria came as the secretary of state, visiting the middle east, hinted that the United States might be about to step up its aid to Syrian rebels, perhaps expanding the range of support it provides. Current only non-lethal aid is provided by the United States, although the New York Times among others has reported that U.S. intelligence is working closely with Arab security agencies that are arming certain elements of the loose rebel alliance.

“Yes, I will be meeting with the Syrian opposition in London, and, yes, we will be discussing various means of having an impact on President Assad’s calculations about where the battlefield is going,” Mr. Kerry told reporters Tuesday.

He said he would “leave it to the White House … to make any announcements with respect to any stepped-up efforts.”

“I’m not sure what the schedule is,” he added, hinting at a future policy adjustment, “but … we’ll see what is forthcoming on that in the days ahead.”