WASHINGTON — In an acknowledgment of severe shortcomings in its effort to create a force of moderate rebels to battle the Islamic State in Syria, the Pentagon is drawing up plans to significantly revamp the program by dropping larger numbers of fighters into safer zones as well as providing better intelligence and improving their combat skills.

The proposed changes come after a Syrian affiliate of Al Qaeda attacked, in late July, many of the first 54 Syrian graduates of the military’s training program and the rebel unit they came from. A day before the attack, two leaders of the American-backed group and several of its fighters were captured.

The encounter revealed several glaring deficiencies in the program, according to classified assessments: The rebels were ill-prepared for an enemy attack and were sent back into Syria in too small numbers. They had no local support from the population and had poor intelligence about their foes. They returned to Syria during the Eid holiday, and many were allowed to go on leave to visit relatives, some in refugee camps in Turkey — and these movements likely tipped off adversaries to their mission. Others could not return because border crossings were closed.

The classified options now circulating at senior levels of the Pentagon include enlarging the size of the groups of trained rebels sent back into Syria, shifting the location of the deployments to ensure local support, and improving intelligence provided to the fighters. No decisions have been made on specific proposals, according to four senior Defense Department and Obama administration officials briefed on the matter, who spoke on condition of anonymity to discuss confidential planning.