BEIRUT -- When the United States opened its aerial campaign against the Islamic State group in Syria this week, its first salvo also hit an al-Qaida cell it says was planning terror attacks -- a move that has injected more chaos into the conflict and could help President Bashar Assad.
Amid fears they could be targeted next, two rebel factions already have evacuated their bases, and residents in areas under the control of other Islamic brigades cower at home, wondering whether their districts will be hit.
While al-Qaida's branch in Syria, known as the Nusra Front, is considered a terrorist group by the United States, among the Syrian opposition it has a degree of support and respect because its fighters are on the front lines alongside other rebels battling Assad's forces.
To them, the U.S. strikes, which hit several Nusra Front facilities and killed dozens of its fighters, appeared to signal an American move to take out any rebel faction that adheres to an Islamic ideology -- a large segment of the rebellion against Assad.
U.S. officials say the strikes were aimed at a cell of hardened jihadis within the Nusra Front called the Khorasan Group, which Washington says poses a direct and imminent threat to U.S. and Western interests.
On Thursday, FBI director James Comey acknowledged that the U.S. did not have precise intelligence on where or when the group might attack, adding that there was no indication the airstrikes had disrupted the cell's plots.
"It's hard to say whether that's tomorrow, three weeks from now or three months from now. But it's the kind of threat you have to operate under the assumption that it is tomorrow," Comey told reporters in Washington.
U.S. intelligence officials say the group has been trying to perfect a non-metallic bomb that can get past airport security and be used to blow up an airplane in flight.
But many in the Syrian opposition are skeptical of the U.S. claims and believe the airstrikes simply aimed to hurt the Nusra Front -- and by extension the anti-Assad uprising. The Khorasan Group -- a name given the cell by American officials -- was unheard of publicly less than a month ago.
"I don't think it's ever been a separate group on the ground," said Aymenn al-Tamimi, an expert on Syrian and Iraqi militants. "I think the problem for the U.S. is that in wanting to target Nusra, there's still this problem that Nusra has local support and there are still many rebel groups that work with Nusra."
While U.S. and Western officials view both the Islamic State group and the Nusra Front as a threat, on the ground, Syrians make a huge distinction between the two -- despite their shared history.
The Nusra Front was created with financing, manpower and military hardware provided by the Islamic State group when the Islamic extremists were still known merely as the Islamic State in Iraq. The Nusra Front and its patron eventually had a falling out in 2013 for ideological as well as strategic reasons.
The Nusra Front, while loyal to al-Qaida, has cooperated with other Syrian rebel factions in the fight to oust Assad. The Islamic State group, on the other hand, focused not on Assad but rather on creating its version of a medieval Islamic state -- and was happy to battle all comers, government and rebels, to achieve that goal.
Now, the Islamic State group controls a vast tract of land stretching from the Turkish border in northern Syria to the western outskirts of Baghdad, where it has declared a self-styled caliphate, or Islamic state, ruled by its brutal version of Islamic law. Its aggressive push across Iraq in June spurred the U.S. to gather an international coalition to try to defeat the extremists.
The Nusra Front, meanwhile, has seen its fortunes fade. The al-Qaida affiliate is reportedly struggling with its finances, and has shed fighters as its clout has waned. It remains locked in battle with the Islamic State group in Syria as well as Assad's forces -- all the while fighting arm-in-arm with some Western-backed groups against both.
It is that cooperation with other rebel groups that could be undermined by the U.S. airstrikes, said Aron Lund, editor of the Carnegie Endowment's Syria in Crisis report. He noted that the U.S. would likely have had to have informants on the ground -- such as activists or Western-backed rebels -- to pinpoint facilities belonging to the Nusra Front in order to target them.
"You have the potential, I think, for the Nusra Front to sort of react defensively to this by attacking or forcing out rebel groups that they feel will work with the U.S. in targeting them," Lund said.
That could trigger a bout of infighting that the anti-Assad movement can ill afford at a time when it is already under pressure from fighting a two-front war against the government and the Islamic State.
Another source of instability in the rebel ranks stems from concerns among other Islamic rebel brigades -- and there are many -- that U.S. airstrikes could target them as well.
On Wednesday, Ahrar al-Sham, a powerful ultraconservative rebel group that has been among the most effective forces fighting to oust Assad, began evacuating its bases in northern Syria. The group issued a statement calling for its fighters to limit the use of wireless communication devices to emergencies, to move heavy weapons and conceal them, and to warn civilians to stay away from the group's camps.
For its part, the Nusra Front has packed up its camps in Idlib province and decamped to try to avoid being hit again.
The reverberations of the U.S. decision to strike the al-Qaida affiliate were being felt as far away as the opposition-held Damascus suburb of Douma, where rebels have held out against relentless shelling from Syrian government forces.
"Does the coalition think the Islamic Union in Damascus is a terrorist? Is it going to be bombed?" activist Hassan Taquleden asked worriedly, referring to his small rebel faction.
"Residents are terrified that they will be bombed," Taquleden said via Skype. "Honestly, we are barely handling the strikes by Assad. It would be a disaster if the coalition hits here, even with the pretext of helping" moderate rebels.
In Damascus, an activist who goes by the name of Abu Akram al-Shami said U.S. strikes against Nusra Front and other Islamic factions are "against the Syrian revolution and everything we worked for," noting that the most powerful armed groups fighting Assad's forces were the Islamic brigades.
The jolt from targeting the Nusra Front has provided an opening the Syrian government might exploit, Lund warned.
"Militarily, if these groups are weakened or coordination breaks down, because you've had coordination between groups that now suddenly start getting suspicious of each other or want to move away from Nusra because Nusra is targeted, I'm sure that could help the regime in many ways," he said.
-- Associated Press writer Ken Dilanian in Washington contributed to this report.