This photo released June 28 by the New Syrian Army shows fighters of the U.S.-backed group in an undisclosed place in Syria. (New Syrian Armyvia AP)

The U.S. military’s efforts to confront the Islamic State in Syria suffered another setback Wednesday after the militants routed the only group to have survived intact an ill-fated Pentagon program to train and equip moderate rebels last year.

The U.S.-backed New Syrian Army said it was forced to withdraw its forces to its base at Tanf near the Jordanian border after launching what appears to have been a poorly conceived offensive aimed at capturing the strategically important eastern Syrian town of Abu Kamal on the Syrian-Iraqi border.

Islamic State claims published by its Amaq news agency that its fighters had killed 40 members of the group and captured 15 could not be independently confirmed and appeared to be exaggerated. Islamic State social media accounts posted photographs and videos showing brutalized bodies, the beheading of one fighter and small quantities of captured, U.S.- supplied weaponry.

The New Syrian Army said in a statement only that it lost “several men” before the group “successfully departed” to Tanf, more than 150 miles away in remote desert terrain near the Jordanian and Iraqi borders.

Abdulsalem Muzil, a spokesman for the rebel group with which the NSA is affiliated, said the fighters retreated with most of their weaponry and vehicles intact. He called the operation a success.

“The whole operation was a test of power for the New Syrian Army, and our forces proved they can fight ISIS,” he said, using an acronym for the Islamic State.

But other Syrians affiliated with the group said its attack did not go according to plan and that sleeper cells in the town that were expected to join the offensive failed to materialize.

The battle was a setback, they said, for a small group that was depleted further by an Islamic State suicide bomber in May and by a Russian airstrike in June. The group was formed last year with only about 100 men, and its ranks had dwindled significantly by the time the offensive was launched, according to commanders.

The rout also raised further questions about the United States’ strategy in Syria at a time when the Islamic State’s defenses are crumbling on multiple fronts elsewhere across its self-proclaimed caliphate. On Monday, Iraqi security forces finished driving the militants from the symbolically important town of Fallujah 45 miles west of Baghdad, and in recent weeks the Islamic State has been forced to retreat in several key locations of northeastern Syria by the predominantly Kurdish Syrian Democratic Forces (SDF).

The eastern Syrian province of Deir al-Zour, where Abu Kamal is located, is now the only major territory controlled by the Islamic State in which the militants face no challenge. The offensive could have changed that, while also severing one of the Islamic State’s last remaining supply routes between Syria and Iraq.

The New Syrian Army could potentially pose a threat to Islamic State control of the province, because its members are from the area, most of them from Abu Kamal. Although the group had hoped that the attack might trigger a local uprising, the result instead seemed to hand the Islamic State a propaganda victory, even if its claims were exaggerated, analysts said.

Unless the New Syrian Army can regroup soon and launch another offensive, “they will be finished,” said Michael Horowitz, a senior analyst at the Levantine Group security consultancy. And if it is, he said, U.S. strategy will become even more reliant on Syria’s Kurds, who have proved to be an effective fighting force but are regarded with deep distrust by most of the Syrian Arabs living in areas still controlled by the Islamic State.

“If this is the end of the offensive, it is a major setback for U.S. policy in Syria and any future attempts to build an Arab force,” he said. “It will lead to reinforcement of U.S. support for the SDF, which is mostly made up of Kurds, and we know this could be a problem because they have their own interests.”

Zakaria Zakaria in Istanbul contributed to this report.