WASHINGTON — A small number of C.I.A. officers are operating secretly in southern Turkey, helping allies decide which Syrian opposition fighters across the border will receive arms to fight the Syrian government, according to American officials and Arab intelligence officers.

The weapons, including automatic rifles, rocket-propelled grenades, ammunition and some antitank weapons, are being funneled mostly across the Turkish border by way of a shadowy network of intermediaries including Syria’s Muslim Brotherhood and paid for by Turkey, Saudi Arabia and Qatar, the officials said.

The C.I.A. officers have been in southern Turkey for several weeks, in part to help keep weapons out of the hands of fighters allied with Al Qaeda or other terrorist groups, one senior American official said. The Obama administration has said it is not providing arms to the rebels, but it has also acknowledged that Syria’s neighbors would do so.

The clandestine intelligence-gathering effort is the most detailed known instance of the limited American support for the military campaign against the Syrian government. It is also part of Washington’s attempt to increase the pressure on President Bashar al-Assad of Syria, who has recently escalated his government’s deadly crackdown on civilians and the militias battling his rule. With Russia blocking more aggressive steps against the Assad government, the United States and its allies have instead turned to diplomacy and aiding allied efforts to arm the rebels to force Mr. Assad from power.