BAGHDAD -- The lone Syrian rebel group with an explicit stamp of approval from Al Qaeda has become one of the uprising's most effective fighting forces, posing a stark challenge to the United States and other countries that want to support the rebels but not Islamic extremists.
Money flows to the group, the Nusra Front, from like-minded donors abroad. Its fighters, a small minority of the rebels, have the boldness and skill to storm fortified positions and lead other battalions to capture military bases and oil fields. As their successes mount, they gather more weapons and attract more fighters.
The group is a direct offshoot of Al Qaeda in Iraq, Iraqi officials and former Iraqi insurgents say, which has contributed veteran fighters and weapons.
"This is just a simple way of returning the favor to our Syrian brothers that fought with us on the lands of Iraq," said a veteran of Al Qaeda in Iraq, who said he helped lead the Nusra Front's efforts in Syria.
The United States, sensing that time may be running out for Syria's president, Bashar al-Assad, hopes to isolate the group to prevent it from inheriting Syria or fighting on after Mr. Assad's fall to pursue its goal of an Islamic state.
As the United States pushes the Syrian opposition to organize a viable alternative government, it plans to blacklist the Nusra Front as a terrorist organization, making it illegal for Americans to have financial dealings with the group and most likely prompting similar sanctions from Europe. The hope is to remove one of the biggest obstacles to increasing Western support for the rebellion: the fear that money and arms could flow to a jihadi group that could further destabilize Syria and harm Western interests.
When rebel commanders met Friday in Turkey to form a unified command structure at the behest of the United States and its allies, jihadi groups were not invited.
The Nusra Front's ally, Al Qaeda in Iraq, is the Sunni insurgent group that killed numerous American troops in Iraq and sowed widespread sectarian strife with suicide bombings against Shiites and other religious and ideological opponents. The Iraqi group played an active role in founding the Nusra Front and provides it with money, expertise and fighters, said Maj. Faisal al-Issawi, an Iraqi security official who tracks jihadi activities in Iraq's Anbar Province.
But blacklisting the Nusra Front could backfire. It would pit the United States against some of the best fighters in the insurgency that it aims to support. While some Syrian rebels fear the group's growing power, others work closely with it and admire it -- or, at least, its military achievements -- and are loath to end their cooperation.
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.
Even antigovernment activists who are wary of the group -- some deride it as "the Taliban" -- said the blacklisting would be ineffective and worsen strife within the uprising. To isolate the group, they say, the United States should support mainstream rebel military councils and Syrian civil society, like the committees that have sprung up to run rebel-held villages.
The Nusra Front is far from the only fighting group that embraces a strict interpretation of Islam. Many battalions have adopted religious slogans, dress and practices, in what some rebels and activists call a pragmatic shift to curry favor with Islamist donors in Persian Gulf countries. One activist said he had a fighter friend with a fondness for Johnnie Walker Black who is now sporting a beard to fit in.
Not all religiously driven rebel groups embrace the Qaeda vision of global jihad, the International Crisis Group said in a recent report. Some have criticized the Nusra Front as serving the interests of the Assad government, which seeks to paint its opposition as terrorists and foreigners.
The Nusra Front is the only Syrian rebel group explicitly endorsed by Al Qaeda in online forums, the report said.
The group gained prominence with suicide bombings in Damascus and Aleppo in early 2012 that targeted government buildings but caused heavy civilian casualties. It was the first Syrian insurgent organization to claim responsibility for suicide and car bomb attacks that killed civilians.
Many of its members -- Syrians, Iraqis and a few from other countries --fought in Iraq, where the Syrian government helped funnel jihadis to battle the American occupation.
In Iraq's Diyala Province, a former member of Al Qaeda in Iraq said that a leader and many members of the group were fighting in Syria under the Nusra Front's banner. An Iraqi security official there said they travel through Iraqi Kurdistan and Turkey to Syria.
"They are well trained mentally and militarily," Major Issawi, the official in Anbar, said. "They are so excited about the fighting in Syria. They see Syria as a dream coming true."
Syrian fighters also have Iraq experience. Abu Hussein, a commander of the Tawhid and Jihad brigade, which is not slated for American blacklisting and has taken a leading role in many battles, said he fought with Al Qaeda in Iraq for six years.
"I decided to return to Syria because our people need me," he said, adding that his group was attracting secular young men because it could provide ammunition, training and medical care that non-jihadist groups could not.
A 35-year-old Syrian musician who gave his name as Hakam said he decided to join an Islamist fighting group because he saw how well it planned and fought and "how determined and professional they are."
He said that he had rarely prayed and had been a drummer in a casino -- he apologized for mentioning the word, which had become distasteful to him -- but that now he was pious and newly disciplined. He said that the group's goal was an Islamic state in Syria ruled by strict Sunni Muslims, and that it would fight any secular government.
"Our mission won't end after the fall of the regime," he said.
Some Syrians have complained of Nusra fighters trying to impose religious strictures on others. But Brian Fishman, a fellow at the Combating Terrorism Center at West Point, said that the Nusra Front appeared to have learned from the mistakes of Al Qaeda in Iraq, which alienated Iraqis with its sectarian attacks and grisly beheading videos.
The Nusra Front appears to be refraining from attacking other Syrian groups, with the exception of clashes with Kurds in the north, where some rebels believe a major Kurdish militia sides with the government.
Thamir al-Sadi, an Iraqi from Diyala who joined the regular Free Syrian Army, said that would change, predicting infighting after Mr. Assad's fall.
"After the fall of Bashar there will be so many battles between these groups," he said. "All the groups will unite against al-Nusra. They are like a snake that is spreading its poison."
Tim Arango reported from Baghdad, and Anne Barnard and Hwaida Saad from Beirut, Lebanon. Reporting was contributed by Hania Mourtada from Beirut; Duraid Adnan and Yasir Ghazi from Baghdad; employees of The New York Times from Mosul, Iraq, and the provinces of Anbar and Diyala; and Michael R. Gordon from Dublin.world
This article originally appeared in The New York Times.