Drone footage taken in Aleppo, Syria shows the vast destruction the city has endured for more than five years. (Reuters)

As rebel-held sections of Aleppo crumbled under Russian bombing this month, the Obama administration was secretly weighing plans to rush more firepower to CIA-backed units in ­Syria.

The proposal, which involved weapons that might help those forces defend themselves against Russian aircraft and artillery, made its way onto the agenda of a recent meeting President Obama held with his national security team.

And that’s as far as it got. Neither approved nor rejected, the plan was left in a state of ambiguity that U.S. officials said reflects growing administration skepticism about escalating a covert CIA program that has trained and armed thousands of Syrian fighters over the past three years.

The operation has served as the centerpiece of the U.S. strategy to press Syrian President Bashar al-Assad to step aside. But U.S. officials said there are growing doubts that even an expanded version could achieve that outcome because of Moscow’s intervention. Obama, officials said, now seems inclined to leave the fate of the CIA program up to the next occupant of the White House.

If so, Obama’s successor will inherit an array of unattractive options. Critics of the proposal to increase arms shipments warn that it would only worsen the violence in Syria without fundamentally changing the outcome. But inaction has its own risks — increasing the likelihood that Aleppo will fall, that tens of thousands of CIA-backed fighters will search for more-reliable allies, and that the United States will lose leverage over regional partners that until now have refrained from delivering more-dangerous arms to Assad’s opponents.

The proposed expansion of the agency program — dubbed “Plan B” because it was seen as a fallback for failed diplomatic efforts — still has supporters, including CIA Director John Brennan and Defense Secretary Ashton B. Carter. But even former ardent proponents, including Secretary of State John F. Kerry, have voiced skepticism about any escalation at this point. He and others fear that the new weaponry could end up killing Russian military personnel, triggering a confrontation with Moscow.

One senior U.S. official said that it is time for a “ruthless” look at whether agency-supported fighters can still be considered moderate, and whether the program can accomplish anything beyond adding to the carnage in Syria.

The CIA units are “not doing any better on the battlefield, they’re up against a more formidable adversary, and they’re increasingly dominated by extremists,” said the U.S. official, who, like others, spoke on the condition of anonymity to discuss a sensitive operation. “What has this program become, and how will history record this effort?”

Backers of the program said that the CIA effort had succeeded in important aspects of its mission — building a politically moderate force that by last year posed a serious threat to Assad. A U.S. official said that the CIA-backed opposition — widely known as the Free Syrian Army — remains largely intact after a year of Russian pounding, and is the only force in Syria capable of prolonging the war and possibly pushing Moscow to abandon Assad as part of a political solution.

“The FSA remains the only vehicle to pursue those goals,” said a second U.S. official.

The White House and CIA declined to comment. Administration officials familiar with Obama’s thinking said all options remain on the table, though the president has made clear his reluctance to use overt military force.

“We continue to press for options that will decrease violence in Aleppo and alleviate the suffering of the Syrian people,” a senior administration official said. “We and our partners will continue to provide support to the opposition and Syrian civil society in a manner that advances those objectives.”

Neither Democratic presidential nominee Hillary Clinton nor her Republican counterpart, Donald Trump, has publicly outlined a position on the widely known but nevertheless classified CIA operation in Syria.

In their final debate, Clinton struck a more hawkish tone, reiterating her support for carving out an area in northern Syria for civilians and moderate opposition elements where Syrian and Russian planes would not be allowed to fly. “A no-fly zone can save lives and hasten the end of the conflict,” Clinton said, adding that doing so would “take a lot of negotiation. It would also take making it clear to the Russians and the Syrians that our purpose here was to provide safe zones on the ground.”

Trump did not articulate specific plans for Syria, other than to describe the war as a disaster and declare that Aleppo — a major city with the largest concentration of opposition forces — is in his view already a lost cause.

U.S. officials said predictions of Aleppo’s imminent fall should be viewed with skepticism. It is more likely, they said, that the battle for Aleppo will drag on for months. Even if it were to fall, some in the administration hope that rebels can open new fronts against the regime in other parts of the country, forcing Russia to spread its air assets more widely.

Clinton was a backer of CIA intervention in Syria when the plan was first proposed in 2012 by then-CIA Director David H. Petraeus and she was serving as secretary of state. But Russia was not directly involved in the conflict at the time, and it is unclear whether Clinton would continue to favor an aggressive agency arms program given Moscow’s powerful presence there now.

“It’s a fine mess we’ve gotten ourselves into,” said a former senior administration official who was directly involved in the early White House deliberations over the CIA program. “There’s a huge risk here since the Russians entered.  . . . The lesson out of this is that if you don’t take action early on, you should almost expect the options to get worse and worse and worse.”

The former official said that Obama now had “understandable reason for caution” but dismissed the White House argument that its inaction on Plan B shouldn’t be interpreted as significant. “The lack of a decision is a decision,” the former official said.

Members of the Free Syrian Army and other U.S.-backed groups in Aleppo said they have gone long stretches without weapons deliveries but have stockpiled arms in large quantities since 2014, anticipating that the air bombardment would eventually give way to a ground assault.

Molham Ekaidi, deputy commander of an FSA unit in Aleppo, said in an online interview that the United States’ failure to deliver advanced antiaircraft weapons to aid in the defense of Aleppo amounted to a “green light” for Moscow to lay waste to the city.

U.S. intelligence officials say that the rebels have proved to be effective street fighters but that they aren’t sure how long they will be able to hold out given the extensive damage inflicted from the air. Ekaidi said street fighting would favor the rebel side.

“They won’t be able to solve Aleppo by military means,” Ekaidi said. “The regime is weak when it comes to street warfare. The air bombardment won’t be effective enough.”

Obama was always lukewarm in his enthusiasm for CIA intervention. In 2012, he commissioned a classified study of other cases of the agency backing rebel forces. In an interview with the New Yorker magazine, Obama said that he wanted examples of when “that actually worked out well. And they couldn’t come up with much.”

When the deteriorating situation in Syria prompted Obama to authorize the CIA to begin vetting, training and arming moderate factions in 2013, he imposed constraints that frustrated agency operatives. Their goal in Syria would not be to enable rebels to win and seize power, according to officials’ accounts, but to push the conflict toward a stalemate and force various factions to negotiate Syria’s future after Assad.

The CIA set up jointly run compounds in Jordan and Turkey, where officials said more than 10,000 rebels have gotten training and equipment over the past three years. Those vetted units are part of a constellation of opposition groups with 50,000 or more fighters that have gotten money and weapons from the CIA and regional partners including Saudi Arabia, Qatar and Turkey.

The terms required the partners to keep certain classes of weapons out of Syria, particularly MANPADs, highly portable surface-to-air missiles that Washington worried would fall into the hands of terrorist groups and be used to target civilian aircraft.

Rebels chafed at the restriction, complaining that it left them vulnerable to air attack by Assad and, more recently, Russia. The Plan B proposal envisioned a compromise in which the CIA and its partners would deliver truck-mounted antiaircraft weapons that could help rebel units but would be difficult for a terrorist group to conceal and use against civilian aircraft.

As the Russian pounding of Aleppo intensified, horrific images of injured children and destroyed hospitals put new pressure on Obama to authorize expanded weapons shipments to besieged opposition groups. Plan B was raised during a series of weekly White House meetings and was finally put to Obama during an Oct. 14 session with the National Security Council.

Carter has for months favored a “doubling down” of the CIA program, officials said, to inflict higher costs on Moscow for its intervention, while opposing using U.S. military force out of worry that it would divert resources from the campaign against the Islamic State.

But he and Brennan have been outnumbered by skeptics. White House Chief of Staff Denis McDonough has been cautious about the operation from the outset. Kerry, a longtime supporter, has edged toward the camp of doubters, officials said, partly out of concern that any U.S. escalation at this time will only trigger an asymmetric response by Moscow.

“The Russians have seized the initiative,” said a second senior administration official involved in Syria discussions. “You can’t pretend you can go to war against Assad and not go to war against Russia.”

The CIA’s own assessments of the program have been viewed with suspicion by some at the White House, officials said. “Does it make any sense that the people who are totally invested in this program . . . are the same people who are writing analyses of the Syrian opposition on which decisions are based on the future of that program?” the first U.S. official said.

Amid the setbacks in Syria in recent months, key figures in the administration have advocated prioritizing the fight against the Islamic State, rather than against the Assad government. But agency officials disagree with this rationale, saying that the Islamic State can’t be eradicated until a new government emerges capable of controlling the terrorist group’s territory in Raqqa and elsewhere.

“You can’t defeat ISIL without removing Assad,” the second U.S. official said, using an alternative name for the Islamic State. “As long as there is a failed state in Syria, ISIL will have a homeland.”

Obama’s reluctance has frustrated the CIA’s partners overseas who were expecting Plan B to be approved. U.S. officials said the ban on the most worrisome weapons remains intact. Key partners such as Turkey, with busy airports and a long border with Syria, are equally determined to keep MANPADs and other munitions out of extremists’ hands.

Still, a senior Turkish official said his government feels misled about U.S. intentions and would likely begin exploring unilateral arrangements to provide heavier arms to Turkey-backed groups.

“They promised to give more support,” the Turkish official said. “But it now seems like nothing is going to happen. This coalition hasn’t delivered. It’s obsolete now. So we’re going to look at our options. If Aleppo falls, Assad wins.”

Greg Jaffe contributed to this report.