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As a result, the official said, the policy of moving aid to the rebels must be implemented “carefully and appropriately,” with materials going only to “vetted individuals” and “vetted units” fighting on the ground in Syria.

How to successfully conduct such vetting remains at the core of a debate that has long divided some of the Obama administration’s most senior foreign policy and national security officials. Details remained vague Thursday. The senior State Department official, who spoke on the condition of anonymity, implied that Mr. al-Khatib’s coalition, which is based in Cairo, would play a significant role.

The official declined to comment on the extent to which U.S. military forces may be involved in directly training Syrian rebels.

An estimated 70,000 Syrians have been killed in fighting since military forces loyal to Mr. Assad began cracking down on opposition groups in March 2011. Despite the Obama administration’s public resistance to flooding the conflict with weapons, news reports have suggested that the White House and CIA have collaborated with Turkey, Qatar and Saudi Arabia, which are secretly channeling weapons to the rebels.

A report this week by The New York Times cited unidentified “American and Western officials” as saying that Saudi Arabia has financed a large purchase of infantry weapons from Croatia and quietly funneled them to the rebels.