A missile-loaded Turkish air force jet after taking off from Incirlik Air Base in southern Turkey on Wednesday. After months of reluctance, Turkish warplanes last week started striking militant targets in Syria and agreed to allow the United States to launch its own strikes from Turkey. (Emrah Gurel/AP)

The commander of a U.S.-backed Syrian rebel group has been captured by al-Qaeda militants near the spot north of Aleppo where a new contingent of U.S.-trained Syrian opposition fighters entered the country earlier this month from Turkey, the group said Thursday.

Nadim al-Hassan, whose fighting unit is known as Division 30, was picked up with a number of his forces in northwestern Syria by Jabhat al-Nusra, al-Qaeda’s Syrian affiliate, which has been active in the area along with a range of other opposition forces.

Pentagon officials said that none of the approximately 60 U.S.-trained fighters under Hassan’s command were among those captured. “I can tell you that the New Syrian Force personnel that are a part of our training program are all present and accounted for and none have been detained or captured,” said Navy Capt. Jeff Davis, a Defense Department spokesman.

But a U.S. official familiar with the training program, who was not authorized to speak publicly on the issue, said that the capture was “definitely a setback” for U.S. and Turkish plans to use Division 30 fighters to help direct airstrikes against Islamic State forces and establish a rebel-controlled safe zone in the area.

Details of the incident were scant and confusing. Officials have said that the U.S. trainees, dubbed the New Syrian Force, were drawn from and returned to Division 30. “We’ve had regular contact with him,” the official familiar with the program said of Hassan. “He’s certainly an ally.” But officials denied that Hassan himself was among those trained.

In a statement Thursday, a Division 30 spokesman asked for Jabhat al-Nusra to return Hassan and the others and indicated it had no quarrel with the group. “The Division supports any side that helps Syria and the Syrians against Daesh,” it said, according to the Associated Press. Daesh is another name for the Islamic State.

A U.S. military official said he did not know why Hassan had been captured. “Allegiances change by the hour sometimes,” he said of armed groups fighting in Syria.

Northwestern Syria has been a swirl of fighting groups, with moderate, U.S.-backed forces, sometimes in alliance with Jabhat ­al-Nusra, battling against the army of Syrian President Bashar al-Assad. Into this mix, the Islamic State has been steadily advancing westward along the Syria-Turkey border.

The United States, as part of a congressionally authorized program, plans to train about 5,000 fighters in Turkey to combat the Islamic State. An initial group of about 60 fighters, trained by U.S. forces in Turkey and given new equipment, vehicles and locating devices, was sent into the border region in mid-July, about the same time U.S. and Turkish officials agreed on a new anti-Islamic State offensive in which American and coalition aircraft would fly from Turkish air bases to push the militants back to the east.

The Assad government, which uses aircraft to attack opposition forces located relatively nearby, was warned by the administration — through a previously used conduit at the Syrian U.N. mission in New York — to stay away from where the trained fighters were inserted, another U.S. official said.

Although the trainees are not officially designated as spotters for the upcoming U.S. air attacks, they were given equipment that would allow them to send Islamic State coordinates to U.S. personnel on the Turkish side of the border. Those personnel would then transmit the information to surveillance aircraft and strike controllers, making strikes faster and more precise than using overhead surveillance alone.

The New Syrian Force fighters crossed the border at Kilis, on the Turkish side, to the Syrian border town of Azaz. At least some of them, the U.S. military official said, are believed to have taken leave after their return to Syria.

Hassan, along with his deputy, was said to have been captured while leaving a commanders meeting in Azaz.

Missy Ryan and Thomas Gibbons-Neff contributed to this report.