With help from the C.I.A., Arab governments and Turkey have sharply increased their military aid to Syria’s opposition fighters in recent months, expanding a secret airlift of arms and equipment for the uprising against President Bashar al-Assad, according to air traffic data, interviews with officials in several countries and the accounts of rebel commanders.

The airlift, which began on a small scale in early 2012 and continued intermittently through last fall, expanded into a steady and much heavier flow late last year, the data shows. It has grown to include more than 160 military cargo flights by Jordanian, Saudi and Qatari military-style cargo planes landing at Esenboga Airport near Ankara, and, to a lesser degree, at other Turkish and Jordanian airports.

As it evolved, the airlift correlated with shifts in the war within Syria, as rebels drove Syria’s army from territory by the middle of last year. And even as the Obama administration has publicly refused to give more than “nonlethal” aid to the rebels, the involvement of the C.I.A. in the arms shipments — albeit mostly in a consultative role, American officials say — has shown that the United States is more willing to help its Arab allies support the lethal side of the civil war.

From offices at secret locations, American intelligence officers have helped the Arab governments shop for weapons, including a large procurement from Croatia, and have vetted rebel commanders and groups to determine who should receive the weapons as they arrive, according to American officials speaking on the condition of anonymity. The C.I.A. declined to comment on the shipments or its role in them.