A Free Syrian Army fighter takes aim from cover in Aleppo near Hanano Barracks, a facility controlled by forces loyal to Syrian President Bashar al-Assad. (MUZAFFAR SALMAN/Reuters)

The first contingent of Free Syrian Army members trained by the United States to fight the Islamic State crossed the border from the training facility in Jordan back into Syria this week, according to a U.S. official familiar with the program.

Unverified pictures on social media showed a convoy of new-looking Toyota pickups carrying American-made heavy machine guns and spare fuel cans headed back into the war-torn country.

The fighters, numbering about several dozen, are a part of the FSA’s 30th Division, according to the U.S. official, who spoke on the condition of anonymity because he was not authorized to discuss the training publicly.

While the Defense Department declined on Wednesday to comment on the whereabouts of newly trained fighters, a spokeswoman said the force was expected to integrate itself with other moderate rebel units to increase their combat effectiveness.

“It is anticipated that new Syrian force personnel will coordinate with other moderate opposition forces to build trust between organizations that are countering ISIL,” Navy Cmdr. Elissa Smith, a Defense Department spokeswoman, wrote in an e-mail, using one of several abbreviations by which the Islamic State is known.

The Pentagon-funded train-and-equip program, of which these fighters are a part, came under scrutiny last week when Defense Secretary Ashton B. Carter told the Senate Armed Services Committee that the program had yielded only 60 fighters since it was launched in May. The program has a budget of $500 million, of which half apparently has been spent.

Carter said that 7,000 potential fighters were undergoing screening but that the number of trained fighters was “much smaller than we had hoped for at this point.”

Because of fears that extremist groups such as the Islamic State will seek to infiltrate the program, the vetting process for the fighters is rigorous. Besides a background check, the screening involves a polygraph test and a counterintelligence interview, according to the U.S. official.

“One of the reasons the numbers are so small is because we refuse to budge off of our screening process,” the official said.

He added that the U.S.-trained fighters “are a mixed bag” and suggested that the new force was small and of mediocre quality because the best opposition rebels were fighting against forces loyal to Syrian President Bashar al-Assad.

U.S.-trained Syrian fighters have to sign a pledge to fight the Islamic State, not Assad’s forces.

However, Army Gen. Martin E. Dempsey, chairman of the Joint Chiefs of Staff, said in an interview that “the feedback is that the recruits are very good.”

As the Syrian civil war grinds through its fourth year, the training program continues to deepen U.S. involvement in the region, where hundreds of airstrikes and occasional special operations raids against the Islamic State have become part of the conflict. Although the U.S.-led coalition has taken measures to roll back gains made by the Islamic State, it has avoided any direct strikes on forces loyal to Assad.

Kurdish armed groups have recently seized a number of key Syrian towns north of the Islamic State’s capital of Raqqa, while forces opposing Assad have regained territory in the northwestern part of the country. Around the city of Aleppo, various armed groups are fighting one another. Among them are Lebanon-based and Iranian-backed Hezbollah, which supports Assad; the Islamic State; and a number of moderate factions.

Dempsey expects the U.S.- trained fighters to limit the Islamic State’s activity in the periphery of Syria, but like other defense officials, he gives no timeline for when the expected yearly quota of 3,000-5,000 fighters will be trained.

“The larger strategic issue is that over time . . . it should become clear that the relative advantage that ISIL has on the ground today, especially in Syria, will begin to erode once we begin to field this force,” Dempsey said.