In designating the al-Nusra Front as a foreign terrorist organization and an offshoot of al Qaeda in Iraq, the U.S. State Department has put flesh on the bones of its policy of discriminating between the "bad" Syrian rebels and the "good" ones. But the decision is getting mixed reviews in Syria.

Rebels in brigades fighting around Aleppo have told CNN that the move is a miscalculation. Their argument goes something like this: "The U.S. and the West in general have given us next to no help while we've witnessed thousands die at the hands of Bashar al-Assad's heavy weaponry and dominance of the skies.

"To add insult to injury, Washington has now proscribed one of the most effective fighting forces among rebel groups. We may not share al-Nusra's worldview, but we need their organizational and battlefield experience."

This has translated into resentment and anger toward the United States on the ground, as people ask: "Al-Nusra are terrorists, but Bashar is not?"

For the United States, the decision is part of a broader effort to support a moderate political opposition in Syria -- one that can be part of an eventual transition to a post-Assad Syria.

Without such a presence -- one that can begin to function as a government in waiting -- U.S. policy-makers fear a prolonged war of attrition that will play into the hands of more radical factions while Syria's humanitarian crisis turns into catastrophe.

Additionally there is concern among U.S. officials that groups like al-Nusra will make common cause with Sunni militants in Iraq, who would like nothing better than a sympathetic rear base across the Euphrates River.

U.S. officials are making the argument that the overall affect of the designation of al-Nusra will be to minimize the group's role in the Syrian opposition while not reducing the fighting capabilities of the rebel groups.

"Al-Nusra Front is one of many groups that are fighting the Syrian regime now, it is not the only one," a senior administration official told reporters Tuesday in explaining the designation. "In fact it is a minority. Its influence has grown in recent months, but it still represents the minority element within the broader armed opposition."

By the estimates of rebel commanders in Syria, al-Nusra (also known as Jabhat al-Nusra) makes up less than 10% of the brigades fighting the regime. Most of its fighters are Syrian. Some waged jihad in Iraq before returning home, though several also spent time in the Bucca prison camp in Iraq, where hundreds of insurgents were held.

There is also more than a smattering of foreign fighters in al-Nusra's ranks -- Jordanian, Iraqi, Libyan, a few from Central Asia.

U.S. officials also point out that the State Department has designated Syrian groups known as the Shabiha -- paramilitary militias controlled by the government of Syrian President Bashar al-Assad that have attacked and killed opponents of the al-Assad regime.

"Whether the American steps today will immediately curtail al-Nusra's capabilities, I don't think they know," the senior administration official said. "But I think that other nations that are involved in helping the armed opposition will now take more seriously our concerns about the Nusra front."

State Department spokeswoman Victoria Nuland said Monday: "We've had concerns that al-Nusra is little more than a front for al Qaeda in Iraq who has moved some of its operations into Syria. ... We do see al Qaeda in Iraq trying to make these inroads."

Some tribal leaders in Iraqi provinces like Anbar are already thought to be funneling money and weapons to Islamist factions across the long border, and al-Nusra has been strong in areas near the Iraqi border.

The West appears ready to accept that Islamists loyal to organizations such as the Muslim Brotherhood will be part of the post-Assad scenery. They are well represented in the new Military Council established this week, from which al-Nusra was excluded. Al-Nusra had already boycotted the new political opposition, making common cause with other militant factions.

Some analysts believe that al-Nusra is too important on the battlefield, especially in attacking military bases in the Northwest, for other rebel commanders to ostracize it.

In the year since al-Nusra emerged, it has carried out a number of complex and coordinated suicide bombings against high-profile regime targets and built a coherent fighting force through a thorough vetting process. By the admission of Free Syrian Army commanders on the ground, al-Nusra was instrumental in the seizure of the Sheikh Suleiman Military Academy near Aleppo this week. A reporter for Agence France-Presse in the area said many of the fighters "were from other Arab countries and Central Asia."

At another military base, rebel commander Ali Jadlan told CNN that fighters with al-Nusra were among the three brigades laying siege to the military academy. He said they were responsible for the most dangerous area.

The group has continued its offensive against the regime with a devastating series of suicide bombings -- most recently claiming responsibility for blowing up a government checkpoint in al Qusayr, and posting before and after photographs on jihadist online forums.