BEIRUT, Lebanon — The Nusra Front, Syria’s largest and most powerful jihadist rebel group, has built a reputation as an effective fighting force in the country’s civil war, even though its extremist ideology — and accusations that it is part of Al Qaeda — have caused friction in Syria and anxiety abroad.

Last month, the United States designated the front as a foreign terrorist organization, saying it was an alias for Al Qaeda in Iraq. It has claimed responsibility for hundreds of actions since it announced its formation a year ago, including suicide bombs and attacks that have led to heavy civilian casualties.

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