Updated Saturday, December 8, 2012 at 10:46 PM
BAGHDAD — The lone Syrian rebel group with an explicit stamp of approval from al-Qaida has become one of the uprising's most effective fighting forces, posing a challenge to the United States and other countries that want to support the rebels but not Islamic extremists.
Money flows to the group, Jabhat Al-Nusra, 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-Qaida in Iraq, which has contributed veteran fighters and weapons, Iraqi officials and former Iraqi insurgents say.
"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-Qaida in Iraq, who said he helped lead Jabhat Al-Nusra's efforts in Syria.
The United States, sensing time may be running out for Syria's president, Bashar Assad, hopes to isolate the group to prevent it from inheriting Syria or fighting on after 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 Jabhat Al-Nusra 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.
Jabhat Al-Nusra's ally, al-Qaida in Iraq, is the Sunni insurgent group that killed numerous U.S. 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 Jabhat Al-Nusra 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 Jabhat Al-Nusra 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 the United States seeks to bolster, expressed exasperation that the U.S., which has refused to provide weapons throughout the conflict that has killed more than 40,000 people, is opposing a group they see as a vital ally.
The Jabhat Al-Nusra "defends civilians in Syria, whereas America didn't do anything," said Mosaab Abu Qatada, a rebel spokesman.
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 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 Islamic militants have seized in Syria.
Even anti-government 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.