A Western European diplomat, who requested anonymity in order to discuss the issue candidly, said the European Union would designate Nusra as a terrorist organization if the United States did so.

“If they are put on the terrorism list by the United States, we will follow. They are part of al Qaida,” the diplomat said.

Asked for comment, State Department deputy spokesman Mark Toner said he had "nothing to announce on this."

Experts and U.S. officials see the move against Nusra as a part of a U.S.-backed effort to strengthen the new National Coalition of Syrian Revolutionary and Opposition Forces. The coalition, formed last month with Western and Arab support, is seeking international recognition as a government in waiting that would replace Assad’s embattled regime, should it fall, and guide Syria’s transition to democracy.

The coalition has struggled to bring under its command the rebel forces that are battling to end four decades of Assad family rule, including the loose amalgam of groups known as the Free Syrian Army.

Nusra and other groups have refused to accept the new coalition’s authority, however, fueling fears that they’ll also refuse to accept civilian rule should the rebels defeat the Assad regime, and will continue fighting for a Taliban-style Islamic state, a position Nusra leaders weren’t shy about voicing to a McClatchy reporter.

“Eighty percent of Syrians want Islamic law,” Iyad al Sheikh Mahmoud, the leader of a recently founded Jabhat al Nusra group in the central Syrian city of Qalat al Mudiq, said last week in explaining why he didn’t think that elections would be necessary if Assad falls.

Secular rebels have complained that the West’s hesitance to provide them with financial aid has fueled Nusra’s success.

“The strongest ones fighting now are the ones who have money," said Abdullah Alsayed, a former Syrian army captain who led a rebel unit before coming to the United States in January. “All the money comes in from Saudi Arabia and Qatar and all the money goes to the Salafis and Islamists," a reference to the religious conservatives who provide Nusra with its fighters.

Some experts warned that declaring Nusra a foreign terrorist organization was likely to hurt the anti-Assad uprising by fueling tensions between the group and other opposition units. The designation could disrupt the coordination behind recent rebel advances and even risk clashes among rebel groups.

“I’m not saying they aren’t a terrorist group. But given the circumstances and given their cooperation with the opposition as a whole, designating them now would be disastrous,” said Elizabeth O’Bagy, an analyst with the Institute for the Study of War who recently returned from touring rebel-held areas to research Nusra and other Islamist groups.

One major problem, analysts explained, is that the United States has little information that allows it to distinguish between the front’s hard core – including many foreigners – wedded to al Qaida’s vision of creating a state based on its rigid interpretation of Islam, and Syrians drawn to the group by its reputation of being the best armed, trained and disciplined rebel unit.

The best way of isolating the hard-liners is to allow other Syrian rebel groups to convince moderate fighters to defect from Nusra, according to O’Bagy, who said she thought that process was already under way.

During a visit to the war-torn city of Aleppo, O’Bagy said, she witnessed a celebration by rebels who “had managed to get an entire battalion of 200 men to drop the Jabhat al Nusra flag” and rejoin their group.

Nusra’s fighters, however, include many who say they fought against U.S. forces with al Qaida’s Iraq branch, and many of its leaders – including the top commander in Syria’s Deir al Zour province – are Iraqis.

Aaron Zelin, an analyst who follows Nusra for his research on militants at the Washington Institute for Near East Policy, said the administration’s plans to designate the group a foreign terrorist organization were unusual because such determinations typically came only after a threat or attempt to strike U.S. interests.

Zelin, who reads and compiles Nusra’s communiques, said that while the group hadn’t made any specific threats against the United States, it had warned, “Don’t try to meddle in Syria or else you’ll pay the price.”

“This is an interesting case because it’s very proactive on the part of the administration,” said Zelin, who blogs about Nusra and other militant groups at Jihadology.net. “At this juncture, Nusra’s only goal is to take out the Assad regime, but once the regime is gone, I’d imagine that parts of the group would try to set up a base there.”

McClatchy special correspondent David Enders in Beirut and Roy Gutman in Istanbul contributed to this report..