Two weeks after US warplanes began bombing Islamic State (Isis) positions in Syria, the Pentagon leadership has yet to make critical decisions about building the proxy rebel force central to its plan for taking territory away from the jihadist army.

US military officials consider raising a Syrian rebel force crucial for the war aim of ultimately destroying Isis without committing US soldiers and marines to another bloody Middle East ground war. But the Pentagon has yet to even assign a US officer to the task of determining which rebels are trustworthy and capable enough to comprise that force.

“No decision has been made as to who will lead the programme,” commander Elissa Smith, a Pentagon spokeswoman, confirmed to the Guardian.

Once selected for the training, the rebels will be led by Major General Michael Nagata, a special-operations veteran. The supposedly moderate Free Syrian Army is expected to form the kernel of the proxy force.

But the Pentagon, which hopes to field an initial force of nearly 5,000 to fight Isis out of its Syrian strongholds, has yet to determine which fighters amid Syria’s panoply of mostly Islamist rebel groups are eligible to receive US cash, heavy weaponry and Nagata’s expertise. Isis commands as few as 20,000 fighters, and as many as 31,000, according to public US estimates.

The Syrian rebels are the soft underbelly of the Obama administration’s strategy against Isis. Pentagon officials routinely concede that air strikes are insufficient to oust Isis from areas in Syria under its control. Yet even under optimistic conditions, the Pentagon does not expect to have units prepared to attack Isis on the ground in Syria for eight months at the earliest.

Questions remain hanging over every aspect of the Syrian rebel effort. The US wants to use “vetted” and “moderate” Syrians against Isis, but those fighters have consistently lost ground to both Isis and loyalists of Bashar al-Assad’s regime. Obama has decried them as an amateurish force of uncertain capability, even after approving a CIA programme to aid some of them.

Many, including the Free Syrian Army, have ties to radical Islamist factions, as the Syrian civil war does not neatly break down into secular and religious categories. A consistent worry inside the Obama administration, one that kept the US from overtly providing weaponry for most of the war, is that US artillery and the expertise to operate it will ultimately end up in the hands of current or future adversaries. Much of Isis’s heavy weaponry comes from US stocks, captured from overwhelmed Iraqi forces.

Major questions about the Pentagon’s Syrian vetting remain unsettled, including how extensive it will or even can be, given a much-lamented difficulty in cultivating reliable intelligence sources on the ground. Multiple US military officials declined to answer questions surrounding the basic criteria for eligibility in the programme.

Once vetted and trained, the Syrian rebels are being asked to defer their primary goal of ousting Assad for the American goal of ousting Isis. After initially wavering, the Pentagon leadership clarified last week that US aircraft would provide air support for Syrian proxies that Assad may attack, a hesitation that may give the rebels pause.

Lurking over the head of the vetting programme is the possibility of unreliable rebel forces turning their weapons on their US trainers. Those so-called “green on blue” attacks killed as many as 44 US and allied forces in Afghanistan in 2012, and while extensive US attention has tamped down the insider attacks, they have killed a US major general and others this year, most recently on 16 September.

A US defence official who would not speak for attribution said that years of providing non-lethal assistance, such as food and medicine, to the Syrian rebels led them to “know the opposition better than we did two years ago”.

The official said the still-inchoate vetting programme would rely on the input of US intelligence and diplomatic officials as well as “regional partners”, indicating that the Gulf Arab states aiding in the anti-Isis campaign – some of whom have maintained a weapons pipeline to extremist factions – will have a role in helping the US choose which Syrians to back.

“We will vet thoroughly, not just at the beginning of the programme, but also do continuous monitoring,” the official said, noting the US military has “decades of experience” vetting foreign militaries for training.

Defense analysts cited the state of the Syrian training as another piece of evidence that US strategy against Isis is more developed in Iraq than in Syria.

“The US military has much more experience working with the Iraqi military than with Syrian rebels, and is much more comfortable coordinating combat operations. The fact that the US military does not have anyone assigned to vet potential trainees is evidence that the current relationship with the Syrian rebels is inadequate to effectively coordinate combat operations,” said Christopher Harmer, a retired US navy officer and analyst at the Institute for the Study of War.