WASHINGTON -- In an apparent bid to isolate Islamist extremists and bolster a new Western-backed Syrian opposition alliance, the United States is moving to declare one of the most effective Syrian rebel groups a foreign terrorist organization because of its alleged ties to al Qaida.

The State Department originally planned to add the Nusra Front – Jabhat al Nusra in Arabic – to its list of international terrorist groups this week, McClatchy learned. The announcement was postponed, however, as officials discussed how to get the maximum impact from the designation.

The designation now is likely just before the United States and its European and Arab allies meet with leaders of the new opposition alliance at a conference Dec. 12 in Morocco, where a significant aid package for the new alliance is expected to be announced.

The impact of the terrorist designation for Nusra, whose members have been at the forefront of many of the rebels’ most recent victories, remains unclear. Many rebel sympathizers said they were concerned that the designation would make it impossible for rebel groups to coordinate in their fight to topple Syrian President Bashar Assad.

"Many groups labeled by the administration as al Qaida are actually not. What is the reason the U.S. administration is considering it (Nusra) al Qaida? All of our focus is on getting rid of the Assad mafia. We welcome anyone in the fight against Assad,” said Radwan Ziadeh, the executive director of the Washington-based Syrian Center for Political and Strategic Studies. “We have had very, very little or no support from the United States. We got promises, promises.”

Nusra first made its mark by claiming responsibility for a series of car and suicide bombings in Damascus that killed dozens last January and that U.S. officials later said bore the mark of the group al Qaida in Iraq. Since then, Nusra has become essential to the rebels’ battlefield operations.

A McClatchy reporter who spent most of last month in rebel-held territory encountered Nusra fighters at several major clashes, including battles in Aleppo, the seizure of a border crossing at Ras al Ayn and the capture of an artillery base in the city of Mayadeen. The pattern appeared to be that the group and other jihadist units would capture territory, then turn it over to secular rebels who fight under the Free Syrian Army name. That type of coordination might be complicated by a terrorist designation, raising the specter of a scenario in which a U.S.-approved rebel group is working with a U.S.-labeled terrorist group.

U.S. law enacted after the Sept. 11, 2001, terrorist attacks freezes the U.S. assets of individuals or groups that are placed on the U.S. international terrorism list and prohibits American citizens and U.S. residents from making financial transactions with them.

The symbolism of the move may be more important, however, as the administration tries to build the credibility of the new opposition bloc, which replaced one that the United States and many Syrians viewed as dominated by exiles.

It also appears that the Obama administration is counting on the European Union to follow suit, thereby encouraging other governments to cut off funding and other support for the group and to isolate it politically.

Nusra members in Syria have told McClatchy that most of their funding comes from individuals in Saudi Arabia.