A daring mission by U.S.-backed Syrian rebels to infiltrate and take over an ISIS stronghold ended in calamity Wednesday as the jihadist group not only resisted the assault, but killed several rebels and confiscated their materiel.

The New Syrian Army (NSA), a Pentagon-trained counterterrorism force, dispatched 200 of its 300 fighters into al-Bukamal, a key gateway city on the border between Syria and Iraq where the Euphrates River crosses the frontier.

They briefly had some success in seizing several ISIS-held positions just outside the city before succumbing to the tipped-off and more numerous jihadists, according to NSA commander Khazaal Al-Sarhan.

In an exclusive with The Daily Beast, al-Sarhan, also known as Abu Abboud, described the details of the Bay-of-Pigs-style fiasco, which involved not only U.S.-trained rebels but Iraqi Sunni tribesmen attacking ISIS from al-Qaim, al-Bukamal’s sister city across the border in Iraq.

“There was tiny support from some sleeper cells in Daesh-held al-Bukamal whose mission was limited to directing and guiding the New Syrian Army troops on their way to the city,” Abu Abboud said over the encrypted messaging platform WhatsApp.

“The sleeper cells’ ability was hindered due to a curfew that was imposed by Daesh [the Arabic acronym for ISIS] at the start of the raid,” he said, “and because of the huge number of the groups’ militants who were fully aware of the New Syrian Army’s operation.”

He denied reports that NSA fighters were airdropped behind enemy lines into al-Bukumal or its environs by U.S. helicopters.

For a short time, the NSA fighters did have some success in seizing several ISIS-held villages, including al-Sukariya, al-Hizam al-Akhdar and the defunct Hamdan air base. But ISIS struck back swiftly and furiously and expelled the NSA from each of these territories in a rout.

ISIS’s Amaq media agency claimed that the jihadists killed 40 NSA rebels and captured another 15—a claim Abu Abboud told The Daily Beast was wildly exaggerated.

“We sustained five casualties and none of our men has been captured,” he said. “The report published by Amaq said that Daesh took several reconnaissance drones from the New Syrian Army. I just want to say that this is a total lie because we did not even utilize such items during the battle. Given the fact that the coalition jets were present during it, why should we use a reconnaissance drone?”

Michele R. Rollins, a spokesperson for CENTCOM, told The Daily Beast the operation is “still ongoing” and would not go into specifics about the battle or what kind of U.S. assistance was given to the NSA.

“We know there was a very tough fight around Abu Kamal/Al Bukumal today and the New Syrian Army suffered a setback,” she said, adding that the Pentagon was still trying “to determine the complete results of that fight and a way ahead for the operations in the middle Euphrates River Valley.”

Judging from images and videos uploaded to social media, ISIS managed to capture satellite communications gear and much of the NSA’s weaponry, which Abu Abboud claimed consisted of Russian-made “Duskha” anti-aircraft machine guns, mortars, sniper rifles and laser-guided Konkur anti-tank missiles. It also exhibited U.S.-supplied pickup trucks emblazoned with the NSA logo on the hoods.

Nic Jenzen-Jones, the director of Armament Research Services (ARES), looking at one ISIS-filmed YouTube video, identified a type of ammunition cartridge manufactured at the Lake City Army Ammunition Plant, a U.S. government-owned but contractor-managed facility in Independence, Missouri. Jenzen-Jones said the NSA also evidently carried M16 rifles and M2-type heavy machine guns.

More significant than the spoils was the fallen. ISIS exhibited the bodies of slain NSA fighters on social media; at least one had had his head cut off.

Abu Abboud blamed the defeat principally on the inability of other rebel factions from eastern Syria—the provinces of Deir Ezzor, Raqqa and Hasakah—to join “under one umbrella in this newly launched anti-Daesh operation.” The factions are currently deployed in other parts of the country such as Idlib and Aleppo, and their divided loyalties were, he added, the “principal element that enabled Daesh to recapture the areas that we captured.”

The NSA’s allies in Iraq may have acted both precipitously and insufficiently in their role in the pincer sortie. Abu Abboud said the tribesmen alerted ISIS to the mission, which apparently allowed the jihadists to call in reinforcement convoys from the al-Qaim, Iraq, side of the border.

ISIS, he said, “lured” the NSA into advancing toward the heavily fortified al-Bukamal where it then conducted ambushes and placed snipers effectively to slow down the movements of the NSA soldiers.

But Abu Abboud insisted that the United States hasn’t done enough to bolster his battered mini-army, which a week ago was hit in two back-to-back or “double tap” airstrikes at its base in al-Tanaf, near the Jordanian border, by two Russian Su-24 warplanes, which dropped 500-pound bombs and cluster munitions. U.S. FA-18s had to be scrambled after the first airstrike; the second came after those jets returned to refuel.

“The Pentagon has the will and ability to collect more manpower but it is taking them a very long time to bring this into effect and this negatively impacts those on the ground in Syria,” he said. The coalition should focus its efforts on liberating Deir Ezzor, he said. That city lies further upstream toward Raqqa, the de facto capital of the so-called Islamic State. Not only is this region a major source of the group’s oil revenue, Abu Abboud pointed out, it also is believed to be the safe haven of the self-styled “caliph” of ISIS, Abu Bakr al-Baghdadi.

—With additional reporting from Omar Abu Layla