Free Syrian Army fighters take their positions, close to a military base, near Azaz, Syria.

The U.S. military is poised to boost its supply runs to rebels fighting ISIS militants in northern Syria, a U.S. official said Thursday, days after an initial air drop of ammunition.

Speaking on condition of anonymity, the official told AFP the Pentagon is ready to augment ammunition deliveries with weapons, provided rebels on the ground can prove they are fighting ISIS.

“There will be more deliveries but only if they can demonstrate that they have used it in an effective way against ISIL,” the official said, using an alternate acronym for ISIS.

“As they demonstrate results, the packages will get heavier and U.S. strikes will occur in places that are advantageous to their operations.”

The official described the rebel-arming program as “performance-based.”

“We’ve left the door open to more things to include some weaponry,” he added. “If they fail … if the things fall into the wrong hands, then those particular groups will be cut off.”

U.S.-led coalition forces on Sunday parachuted 50 tons of small-arms ammunition and rockets to rebels fighting ISIS militants.

The move followed the Pentagon’s announcement last week it would halt its much-criticized program to train moderate rebels, and instead focus efforts on equipping pre-screened rebel leaders from groups actively fighting ISIS.

The Pentagon had to scrap the $500-million program after many failed to pass the screening process, and one group gave ammo and other gear to an Al-Qaeda affiliate.

The Pentagon says the latest batch of ammunition has gone to a group called the Syrian Arab Coalition that has for months been fighting ISIS across an arc of territory north of the ISIS stronghold of Raqqa.

The official said the military was confident the SAC had picked up the ammo, and denied reports some of it had gone to Kurdish groups.

Russia on September 30 began its own bombing campaign in Syria. Moscow claims it, too, is targeting ISIS and “terrorists” but the Pentagon says the vast majority of strikes have been against opponents of Syrian President Bashar al-Assad.

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