With help from the CIA, 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 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 show. The airlift 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 CIA in the arms shipments -- albeit mostly in a consultative role, U.S. 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, U.S. 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 U.S. officials speaking on the condition of anonymity. The CIA declined to comment on the shipments or its role in them.
The shipments also highlight the competition for Syria's future between Sunni Muslim states and Iran, the Shiite theocracy that remains Mr. Assad's main ally. Secretary of State John Kerry pressed Iraq on Sunday to do more to halt Iranian arms shipments through its airspace; he did so even as the most recent military cargo flight from Qatar for the rebels landed at Esenboga early Sunday night.
Syrian opposition figures and some U.S. lawmakers and officials have argued that Russian and Iranian arms shipments to support Mr. Assad's government have made arming the rebels more necessary.
Most of the cargo flights have occurred since last November, after the presidential election in the United States and as the Turkish and Arab governments grew more frustrated by the rebels' slow progress against Mr. Assad's well-equipped military. The flights also became more frequent as the humanitarian crisis inside Syria deepened in the winter and cascades of refugees crossed into neighboring countries.
The Turkish government has had oversight over much of the program, down to affixing transponders to trucks ferrying the military goods through Turkey so it might monitor shipments as they move by land into Syria, officials said. The scale of shipments, according to officials familiar with the pipeline and to an arms-trafficking investigator who assembled data on the cargo planes involved, was very large.
"A conservative estimate of the payload of these flights would be 3,500 tons of military equipment," said Hugh Griffiths, of the Stockholm International Peace Research Institute, who monitors illicit arms transfers.
"The intensity and frequency of these flights," he added, is "suggestive of a well-planned and coordinated clandestine military logistics operation."
Although rebel commanders and the data indicate that Qatar and Saudi Arabia had been shipping military materials via Turkey to the opposition since early and late 2102, respectively, a major hurdle was removed late last fall after the Turkish government agreed to allow the pace of air shipments to accelerate, officials said.
Simultaneously, arms and equipment were being purchased by Saudi Arabia in Croatia and flown to Jordan on Jordanian cargo planes for rebels working in southern Syria and for retransfer to Turkey for rebels groups operating from there, several officials said.
These multiple logistics streams throughout the winter formed what one former U.S. official who was briefed on the program called "a cataract of weaponry."
U.S. officials, rebel commanders and a Turkish opposition politician have described the Arab roles as an open secret, but have also said the program is freighted with risk, including the possibility of drawing Turkey or Jordan actively into the war and of provoking military action by Iran.
Still, rebel commanders have criticized the shipments as insufficient, saying that the quantities weapons they receive are too small and the types too light to fight Mr. Assad's military effectively. They also accused those distributing the weapons with being parsimonious or corrupt.
"The outside countries give us weapons and bullets little by little," said Abdel Rahman Ayachi, a commander in Soquor al-Sham, an Islamist fighting group in northern Syria.
He made a gesture as if switching on and off a tap. "They open and they close the way to the bullets like water," he said.
Two other commanders, Hassan Aboud of Soquor al-Sham and Abu Ayman of Ahrar al-Sham, another Islamist group, said that whoever was vetting which groups receive the weapons was doing an inadequate job.
"There are fake Free Syrian Army brigades claiming to be revolutionaries, and when they get the weapons they sell them in trade," Mr. Aboud said.
The former U.S. official noted that the size of the shipments and the degree of distributions are voluminous.
"People hear the amounts flowing in, and it is huge," he said. "But they burn through a million rounds of ammo in two weeks."
Turkish and Saudi Arabian officials declined to discuss the flights or on any arms transfers. The Turkish government has not officially approved military aid to Syrian rebels.
Croatia and Jordan both denied any role in moving arms to the Syrian rebels.