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.

By helping to vet rebel groups, American intelligence operatives in Turkey hope to learn more about a growing, changing opposition network inside of Syria and to establish new ties. “C.I.A. officers are there and they are trying to make new sources and recruit people,” said one Arab intelligence official who is briefed regularly by American counterparts.

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