The murky aftermath of the attack on Syrian rebels last week by al-Qaida-linked militants has raised questions about how the small, ragtag group of U.S.-trained forces was sent into battle and whether the military needs to make adjustments to the program.

Amid reports that some of the newly trained Syrian rebels were captured, one was killed and others are still unaccounted for, U.S. officials acknowledged they may need to rethink how they put what they are calling the New Syrian Forces back into battle.

"Certainly this past week has highlighted some of the challenges associated with fielding New Syrian forces, but it's important to keep in mind that success does not hinge on one fight or one event and we're still in the early phases of implementing this program," said Col. Pat Ryder, Central Command spokesman. "And that we're continually applying lessons learned and working as a coalition" to improve the training and equipping program.

So far, only 54 Syrian rebels have completed the U.S. training. Of those, at least one was killed last week and as many as five were captured by the Nusra Front militants who attacked the New Syrian Forces' compound. U.S. officials have also acknowledged that after the fight, which they said the Nusra Front lost, some of the New Syrian Forces left the area and not all have been accounted for. The officials spoke on condition of anonymity because they were not authorized to discuss the matter publicly.

Asked if any of the 54 had returned to battle, Ryder said he would not divulge details on where individuals may be. He said the New Syrian Forces are not under U.S. command and control. Instead, once they are trained they return to the Syrian rebel groups that the U.S. has been working with.

The training is focused on teaching the Syrian rebels to battle Islamic State militants, and the attack by the Nusra Front, an al-Qaida affiliate, appeared to come as a surprise.

In a hearing last month, Defense Secretary Ash Carter, responding to a question from Sen. Jack Reed, D-R.I., dismissed as unlikely the idea that the U.S. might have to go to the aid of New Syrian Forces if they were attacked.

"I think we have an obligation to do so. You're right. I don't expect that occasion to arise anytime soon," said Carter.

But a succession of incidents last week, including a Nusra attack and two separate, successful kidnappings that targeted the rebels, have made it clear that assumption was wrong.

While military officials say there is no formal review as a result of the attack and its aftermath, one senior U.S. official said it's prudent to take another look at how and where the Syrian forces are put back onto the battlefield in order to make sure they are as successful as possible.

Currently there are two other groups of Syrian rebels going through training, and hundreds of others are being vetted. The vetting process is lengthy and is aimed at ensuring that the forces are willing and able to fight, are not considered insider threats and agree to focus the fight on Islamic State.

Officials would not say what changes they might make in either the training program or the process of returning trainees to the fight.

"The situation on the ground in Syria is incredibly complex, the situation in terms of the various malign elements within Syria makes reinsertion or exfiltration very challenging," said Ryder. "As we observe the situation, as we instruct new recruits, there are certainly things we're taking into account."

In a related matter, the Pentagon on Friday released updated cost figures for the military operations against Islamic State militants in Iraq and Syria. As of the end of July, the U.S. has spent $3.5 billion, or nearly $10 million per day since operations began a year ago. The bulk of the costs are for Air Force operations, mainly airstrikes.