WASHINGTON -- In late 2011, U.S. officials say, battle-hardened commanders of al Qaida’s Iraq branch slipped into Syria to rally jihadists around the fight to unseat President Bashar Assad, espousing a militant Islamism that worried pro-democracy activists.

A year later, the group those Iraqi commanders formed, the Nusra Front, appears to be leading the insurgency against Assad, even coordinating with non-Islamist rebels who still view the militants with suspicion but can’t dispute their superior battle skills.

U.S. officials on Tuesday formally labeled the Nusra Front part of the al Qaida in Iraq terrorist network, a move designed to drive a wedge between Islamist and nationalist rebels and to prevent extremists from carving out a Taliban-style state in a post-Assad Syria. But the move is unlikely to change Nusra’s pre-eminence on the battlefield, according to a senior U.S. official who briefed reporters under the condition of anonymity.

“Whether the American steps today will immediately curtail Nusra’s capabilities, I don’t think they will,” the official said. But the official said he hoped it would discourage those donating to Nusra, including Saudi Arabia and Qatar.

“I think other nations that are involved in helping the armed opposition will now take more seriously our concerns about the Nusra Front and its expanding influence,” the official said. “It is important for countries to understand what Nusra is and what it represents.”

That’s especially timely now as the United States, after months of hesitation, has decided to throw its support to a new coalition of Syrian opposition groups that came together last month under U.S. pressure. President Barack Obama announced the new policy Tuesday in an interview with ABC News’s Barbara Walters in which he declared the National Coalition of Syrian Revolutionary and Opposition Forces to be the legitimate representative of the Syrian people.

“Not everybody who is participating on the ground in fighting Assad are people that we are comfortable with,” Obama said. “There are some who I think have adopted an extremist agenda.”

Whether that will be enough to wean the secular rebels from Nusra is unclear, 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 in Syria.

The declaration that Nusra is a terrorist group hasn’t cut into Nusra’s support, O’Bagy said, and may in fact be bolstering it. Some prominent rebel commanders – including two well-known moderate generals in Aleppo – pledged solidarity with Nusra. Other Syrians have dubbed the U.S. move a “pro-Assad policy.”

“They still see them as an ally in trying to defeat the regime in ways the U.S. is not at all,” she said.

O’Bagy and other analysts said the U.S. must act quickly to counter that view by pledging direct support for those rebel forces that have aligned themselves with the coalition that Obama recognized Tuesday.

Even that may be too late to undercut Nusra, which has become an integral part of the anti-Assad movement. The State Department said that Nusra has claimed some 600 attacks, including at least 40 suicide operations, since November 2011. A McClatchy reporter who traveled throughout Syria for much of last month encountered Nusra units at every battle site he visited.