A Free Syrian Army fighter in combat in 2013. Many soldiers in the Free Syrian Army traveled to Syria’s border with Turkey in hopes of taking part in a U.S. military training program. (Muzaffar Salman/Reuters)

The U.S. military will send its first class of trained Syrian fighters back into Syria in the next several weeks, in what promises to be a key test for the Obama administration’s effort to stand up an effective local force to combat the Islamic State, Pentagon officials said.

A U.S. military official said that American and allied forces are wrapping up training in Turkey for the first group of moderate opposition fighters, numbering fewer than 100. The men will be sent south into Syria by the end of the summer.

Before that occurs, military officials must finalize plans for reinserting the rebels into an active war zone.

But even as training concludes for this initial cohort of fighters, major questions remain about the effect the program will have on Syria’s crowded battlefield. The White House has not yet decided, for example, what sort of support it will provide to the rebels, especially if they come into conflict with forces loyal to Syrian President Bashar al-Assad.

The program, which aims to eventually field up to 5,400 fighters a year, has gotten off to a slow start as U.S. officials struggle to select and screen qualified candidates.

The military official, speaking on the condition of anonymity to discuss a mission that has been shrouded in secrecy, said the low numbers so far have been “disappointing.” But he also said the slow start illustrated the rigor of vetting designed to screen out candidates who are unsuited to the program or may pose a danger to trainers.

“We think the program will be successful in the long run because we give them a qualitative advantage,” the official said, with new combat skills and U.S.-provided weapons.

“That’s the draw of the program — not quantity, but quality,” the official said.

Officials said they have seen no major effort on the part of the Islamic State or other extremist groups to infiltrate the training. Fewer than five candidates in the first screenings were disqualified for deception.

Faysal Itani, a fellow at the Atlantic Council, a Washington think tank, said that even a relatively modest number of fighters could have an impact if they were trained and armed in a superior way, just as fighters from the Lebanese group Hezbollah have had in Syria in recent years.

In recent weeks, Kurdish and Syrian rebel forces have been seeking to claw back territory held by the Islamic State in northern Syria, a battle that U.S. officials hope will choke off the flow of foreign fighters.

A task force, made up of officials from the U.S. military as well as intelligence and law enforcement agencies, has worked in recent months to determine which opposition fighters could be eligible for training from a group of over 6,000 who initially expressed interest. The CIA has its own covert training program.

“It is harder to join the Syria [training] program than it is to join most modern militaries,” the official said. “The vetting process is that extreme.”

Prior to the launch of the first training class, about 400 fighters, most of them from the Free Syrian Army’s 30th Division, traveled to Syria’s border with Turkey in hopes of taking part in the program. In a sign of the challenges facing groups seeking access to U.S. training, some of the men turned back before crossing the border, to avoid a confrontation with another opposition group.

Many candidates were disqualified early on because they were underage or in poor health, or because their names did match those on trainers’ lists. After the fighters reached training facilities, others were sent home due to homesickness, because they wanted to be home for Ramadan, or because they expressed a desire to fight Assad rather than the Islamic State.