WARSAW — Secretary of Defense Ashton B. Carter said Thursday that the United States had “missed an opportunity” when it chose to shift air support from covering a group of U.S.-backed rebels attempting to seize a key border town in Syria to striking a large Islamic State convoy in Iraq.

Carter’s comments, made to reporters while on his way to the biannual NATO summit, was the most candid acknowledgment yet by the Pentagon that the U.S. had failed to adequately support one of its fledgling proxy forces on the ground during the recent operation.

On June 28, the Pentagon-backed group, called the New Syrian Army, drove more than 200 miles from their base on the Syrian-Iraq border to wrest the town of Bukumal from the Islamic State. Despite early gains, including seizing an abandoned airfield on the outskirts of the town, the unit of roughly 100 fighters was repelled the next day when the Islamic State counter-attacked, killing a handful of the rebels and capturing U.S.-supplied weaponry and equipment.

Instead of circling overhead ready to support the fighters that had been promised air support, U.S. bombers were diverted to strike two large columns of Islamic State vehicles fleeing the recently captured Iraqi city of Fallujah in the early morning hours of June 29. U.S. and British aircraft proceeded to join their Iraqi counterparts, who initiated the strikes, in destroying more than 500 vehicles and killing hundreds of fighters.

The bombardment, nearly unprecedented in the two-year conflict against the Islamic State, was reminiscent of the Gulf War’s “Highway of Death.” It is unclear how many civilians might have been killed.

“The allocation of forces is a daily tactical decision that commanders make,” Carter said, defending the decision to shift the U.S. aircraft away from the tenuously positioned Syrian forces to striking Islamic State militants outside of Fallujah.

“We have a lot of air assets and a lot of partners who have air assets, but on any given day they have to go to a certain place at a certain time,” Carter said, after being asked whether the United States has enough aircraft to support multiple operations spread across Iraq and Syria. “But circumstances change.”

Commanders in the New Syrian Army realized that U.S. aircraft had left during the fighting, but were unaware they had been pulled away to attack a more tantalizing target in Iraq.

Muhanned Tallaa, the group’s leader, said despite the bombers’ temporary absence that they returned when he called them “right away.” When the New Syrian Army assaulted Bukamal, U.S. aircraft carried out eight strikes around the town, while on the day they were routed there was only one, according to reports released by the U.S.-led coalition.

The lack of U.S. air support, however, was only one issue that plagued the rebels’ offensive. Additional fighters, in the form of dormant resistance cells spread throughout Bukamal, failed to arrive, said Abdulsalam Muzil, a fighter with a rebel group that formed the New Syrian Army. A radio station, called Bu kamal liberation radio, was purposed to broadcast a call to arms when the New Syrian Army arrived on the outskirts of the town. The call, Muzil said, went unheeded.

Although the rebel forces were pushed out of the town and forced to drive back to their garrison at al-Tanf, the New Syrian Army characterized the attack a success, stating that it was meant to be a raid that penetrated into well-defended Islamic State territory. However, prior to the offensive’s collapse, U.S. military spokesman Col. Chris Garver said the operation was originally meant to “to liberate Bukamal and cut [the Islamic State’s] military supply lines in the Euphrates Valley between Syria and Iraq.”

Garver told reporters Wednesday that some aspects of the operation are ongoing and that it had managed to disrupt Islamic State operations in the area.

Liz Sly contributed to this report from Irbil, Iraq. Lamothe reported while traveling with Carter.