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.