Leaders of the Free Syrian Army, the loose-knit rebel umbrella group that the United States seeks to bolster, expressed exasperation that the United States, which has refused to provide weapons throughout the conflict that has killed more than 40,000 people, is now opposing a group they see as a vital ally.

The Nusra Front “defends civilians in Syria, whereas America didn’t do anything,” said Mosaab Abu Qatada, a rebel spokesman. “They stand by and watch; they look at the blood and the crimes and brag. Then they say that Nusra Front are terrorists."

He added, “America just wants a pretext to intervene in Syrian affairs after the revolution.”

The United States has been reluctant to supply weapons to rebels that could end up in the hands of anti-Western jihadis, as did weapons that Qatar supplied to Libyan rebels with American approval. Critics of the Obama administration’s Syria policy counter that its failure to support the rebels helped create the opening that Islamic militants have seized in Syria.

The Nusra Front’s appeals to Syrian fighters seem to be working.

At a recent meeting in Damascus, Abu Hussein al-Afghani, a veteran of insurgencies in Afghanistan, Iraq and Libya, addressed frustrated young rebels. They lacked money, weapons and training, so they listened attentively.

He told them he was a leader of Al Qaeda in Iraq, now working with a Qaeda branch in Syria, and by joining him, they could make their mark. One fighter recalled his resonant question: “Who is hearing your voice today?”

On Friday, demonstrators in several Syrian cities raised banners with slogans like, “No to American intervention, for we are all Jebhat al-Nusra,” referring to the group’s full name, Ansar al-Jebhat al-Nusra li-Ahl al-Sham, or Supporters of the Front for Victory of the People of Syria. One rebel battalion, the Ahrar, or Free Men, asked on its Facebook page why the United States did not blacklist Mr. Assad’s “terrorist” militias.

Another jihadist faction, the Sahaba Army in the Levant, even congratulated the group on the “great honor” of being deemed terrorists by the United States.