Moscow entered Syria's multi-faceted civil war last month, when it began air strikes against Islamic State jihadists and other "terrorists" fighting against the regime of its ally President Bashar al-Assad.
Washington and Europe, however, accuse Moscow of targeting "moderate" groups that oppose the Syrian leader, rather than IS, and say ousting Assad is key to ending the bloody four-year conflict.
On Monday, EU foreign ministers demanded Russia stop targeting non-jihadist rebels while the UN's peace envoy to Syria said he was heading to Moscow to promote a political end to the conflict.
US forces have carefully vetted the leader of the group, which includes up to 5,000 fighters, said a Baghdad-based spokesman, after heavy criticism of Washington's previous programme to train moderate rebels.
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The Pentagon had to scrap a USD 500-million scheme to equip and instruct thousands of Syrian rebels in Turkey and Jordan after many failed the screening process and one group gave weapons to an Al-Qaeda affiliate.
"Coalition forces conducted an airdrop Sunday in northern Syria to resupply local counter-ISIL ground forces as they conduct operations against ISIL," US Central Command spokesman Colonel Patrick Ryder said, using an alternative acronym for IS.
A US-led coalition has for the past year carried out air strikes on IS, which controls swathes of both Syria and neighbouring Iraq.