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Sami Ibrahim, an activist based in Damascus, said many Syrians support the al-Nusrah Front because they see it as a group that is “fighting the monster that has killed their children and tortured their fathers and mothers.”

“People in Syria say if al-Nusrah is a terrorist group, what about Bashar Assad?” Mr. Ibrahim said in an interview via Skype.

“The people here will love al Qaeda because they say al Qaeda helps them while Barack Obama just makes statements,” he added.

The United States has been reluctant to arm the Syrian opposition out of concern that the weapons could end up in extremists’ hands. While some Persian Gulf countries have funneled weapons to the rebels, the United States has provided around $50 million in nonlethal assistance and nearly $200 million in humanitarian aid.

“The rebels see Jabhat Nusra as an ally,” said Elizabeth O’Bagy, a research analyst at the Institute for the Study of War. “This has been a side effect of the U.S. policy of not offering greater support to other rebel organizations.

“In many ways, the population sees this [terrorist designation] as an attack against them,” she added.

The al-Nusrah Front has been far more effective than many of the other rebel groups, including the Free Syrian Army, which is made up of army defectors. Over the past year, it has claimed responsibility for about 600 attacks, including more than three dozen suicide bombings in major Syrian cities. The high civilian toll in these attacks has eroded some support for the group.

“I don’t like Jabhat Nusra because its operations have resulted in many civilian deaths,” said Abdullah Ali Alsaeyd, who defected from Syria’s army early in the revolution.

Yet the Free Syrian Army maintains a close relationship with the al-Nusrah Front in some parts of Syria, including in the northwestern city of Aleppo, said Mr. Alsaeyd. The groups coordinate operations and share weapons.

The State Department says the al-Nusrah Front is trying to hijack the revolution.

“Through these attacks, al-Nusrah has sought to portray itself as part of the legitimate Syrian opposition, while it is, in fact, an attempt by AQI to hijack the struggles of the Syrian people for its own malign purposes,” said State Department spokeswoman Victoria Nuland.

In addition to the State Department action, the Treasury Department targeted two al-Nusrah Front leaders — Maysar Ali Musa Abdallah al-Juburi, an Iraqi who is the group’s religious and military commander in eastern Syria, and Anas Hasan Khattab, who coordinates the movement of funds and weapons with al Qaeda in Iraq.

“What is important is to understand that extremists fighting the Assad regime are still extremists, and they have no place in the political transition that will come,” said the senior U.S. official.