(Image Credit: Baraa Al-Halabi/AFP/Getty Images)

A key component of the Obama administration's strategy to "degrade and destroy" ISIS in Iraq and Syria is the training and arming of moderate Syrian rebel fighters. But the biggest hurdle might be how the United States would vet those fighters to make sure they can become a viable fighting force able to take on ISIS.

The administration has asked Congress to authorize the training program, as well as the $500 million in funding needed to get the program started.

But the proposed program has raised questions about how it can effectively vet rebel fighters. Since the administration's national security team first discussed proposals to train and equip Syrians in 2012, the concern has been that vetted fighters might take their training and arms to Islamist rebel groups.

For more than a year, the CIA has run a limited training program in Jordan for small numbers of vetted fighters. The military program would expand on that model and train larger numbers of fighters.

The U.S. military is accustomed to training the militaries of partner nations, but it does not have the legal authority to train a nongovernment fighting force. That would require specific congressional authorization, the reason the administration is asking Congress to pass such an authorization.

U.S. officials familiar with details of the proposed program say that it is slated to last for one year from the time that Congress authorizes it and passes the funding request.

A senior administration official revealed Wednesday that Saudi Arabia had agreed to host the train-and-equip program. A U.S. official told ABC News that Saudi Arabia will host multiple training sites for Syrian rebel fighters.

Officials said groups of fighters would cycle through the program to receive training that would last several months.

The plans for the program appear to have grown along with the ISIS threat. When the administration included a $500 million funding request for the program, it was projected to be a "training the trainers" program. Those are military training programs where fighters would go back to Syria and train their fellow fighters.

It appears to have evolved into a broader program. One official said the plans are for several thousand vetted Syrian fighters to receive U.S. training and arms.

Officials acknowledge that the biggest question to be resolved is how the vetting of moderate rebel fighters will be conducted.

For the most part, the rebel fighting force known as the Free Syrian Army has been identified as the most moderate of the many rebel groups in Syria.

But it has suffered setbacks on the battlefield from Syrian government forces and Islamist rebel groups. It is unclear whether it will be the main recipient of the U.S. train-and-equip program.