The U.S. quickly is running out of options to stop the regime and Russia on Syria’s eastern Aleppo, U.S. officials concede, and they fear that abandoned U.S.-backed rebels could increasingly turn to jihadists groups, like al Qaeda, for protection.

In addition, two U.S. officials told the Daily Beast, they fear the defeat of rebels in Syria’s largest city could weaken U.S.-backed groups in other areas around including Idlib, Hama and Latakia.

That is, the collapse of rebel held areas of eastern Aleppo could mean not just a stronger position for Syrian President Bashar al Assad but radical terror groups, the last remaining opposition forces still standing. The fate of Aleppo could be the turning point of the five-year civil war.

“The rebels have been willing to go along with the coalition up until now. But how long can they hold out against a [Russian] assault?” one distraught U.S. official asked

If that happens, it will validate a long standing Russian narrative that U.S. backed rebels are not moderate as the U.S. claims but radical elements seeking to destroy Syria. And forcing such groups toward more radical elements may be the very intent behind their aggressive assault on eastern Aleppo for the last week, which was launched after the collapse of the latest cease fire.

The Russians and the regime are using “brutal tactics to radicalize other side. And that appears to be by design, not a defect,” Daveed Gartenstein-Ross, a senior fellow at the Washington, D.C.-based Foundation for Defense of Democracies.

“This is self-fulfilling Russian propaganda,” the U.S. official said.

U.S. officials believe the assault by the regime and Russia now includes using bunker buster bombs, designed to pierce hardened targets like bunkers, in addition to incendiary weapons and chemical weapons, all part of a campaign targeting hospitals, aide workers, water supplies and food supplies.

According to the Syrian Observatory for Human Rights, roughly 10,000 civilians have been killed by Russian airstrikes since their campaign on behalf of Assad began a year ago on September 30, an intervention that fundamentally shifted the war toward the regime.

According to the World Health Organization, as recently as the week between September 23 and September 30, 338 people were killed in eastern Aleppo, including 106 children.

"The situation really is unfathomable,” Rick Brennan, WHO's head of emergency risk management and humanitarian response, said a U.N. briefing in Geneva Friday.

Moreover, U.S. officials believe thousands of regime forces have moved into the city and started a block by block clearing of destroyed rebel-held areas of eastern Aleppo.

There are conflicting reports about what the regime forces now hold. A Syrian government source told Reuters its troops had captured several government buildings Friday in parts of Aleppo’s Suleiman al-Halabi district, but rebels said those forces have since retreated.

On Sunday, the Syrian Army reportedly urged the opposition to leave, offering safe passage, as they moved their way through the city.

U.S. officials estimate there are “several thousand” rebels in and around Aleppo. But weapons like TOW missiles and AK-47s in rebel hands are no match against the air assault.

On Thursday, U.S. officials began suggesting for the first time that eastern Aleppo could fall into regime hands, in a matter of weeks, citing the assault on food and water supplies and the presumed Russian and/or Syrian strikes on the two largest hospitals in eastern Aleppo Wednesday.

“You can live without a lot of things, but not water,” one defense official explained to The Daily Beast.

Defenders of the rebels note that if rebels have been unwilling to join al Qaeda forces in eastern Aleppo this long, they are not likely to turn to the group now. But Jennifer Cafarella, a Syria planner at the Washington-based Institute for the Study of War, noted there are other groups, short of al Qaeda, that rebels could turn to for support as their territory shrinks. Among them are al Ahrar al Sham, a Salafi jihadist group.

“There is currently a diverse spectrum of groups in Syria,” Cafarella explained to the Daily Beast.

That said, the defeat of weakened moderate opposition forces in Aleppo would result in an "overall change in the character of the opposition" moving forward, Cafarella said.

The Russians charge that there is no real moderate opposition, but rather the United States is depending jihadist elements, like Jahbat Fateh al-Sham, formerly known as al Nusra, an al Qaeda affiliate, to bolster a small number of moderates.

During the ceasefire negotiations two weeks ago, the U.S. “pledged solemnly to take as a priority an obligation to separate the opposition from Nusra," Russian Foreign Minister said Sergey Lavrov told the BBC Friday.

"They still, in spite of many repeated promises and commitments... are not able or not willing to do this and we have more and more reasons to believe that from the very beginning the plan was to spare Nusra and to keep it just in case for Plan B or stage two when it would be time to change the regime,” Lavrov told the BBC.

U.S. officials currently are considering options for eastern Aleppo, in the face of the ongoing bombardment and collapse ceasefire talks between the Russians and the U.S. State Department. But those discussions so far are at a staff level. And with every day of Russian-led aerial bombardments, options dwindle, the U.S. officials conceded.

At least one option remains off the table. Ash Carter, Secretary of Defense, and Marine Gen. Joseph Dunford, the chairman of the Joint Chiefs of Staff, have resisted military options, saying the U.S. military campaign should remain focused at defeating the self-proclaimed Islamic State in Iraq and Syria. The administration has so far agreed with that assessment.

“At the end of the day, there are going to be challenges around the world that happen that don't directly touch on our security, where we need to help - we need to help lead, but just sending in more troops is not going to be the answer,” PresidenT Obama told CNN”s Jake Tapper at a presidential town hall Wednesday with members of the U.S. military and their families.

The result is that the various U.S. government agencies invested deeply divided over U.S. options for what to do in eastern Aleppo, particularly for the CIA, which worked hardest at finding, training and arming U.S.-backed opposition groups.

“If eastern Aleppo falls, it would be a major setback to say the least,” a second U.S. official concluded.