The Syrian Democratic Forces (SDF), an alliance of Washington-backed Kurd and Arab fighters, said Saturday that several of its fighters were injured in a bombing by Russian warplanes east of the Euphrates River near Syria's oil-rich province of Deir Ezzor.
A Russian military spokesman denied targeting the group, though the coalition claimed that the Russians knew the zone contained SDF fighters.
General Joe Dunford, chairman of the US Joint Chiefs of Staff, said he had spoken to his Russian counterpart General Valery Gerasimov and that Secretary of State Rex Tillerson had also addressed the issue with Russian Foreign Minister Sergei Lavrov.
"I spoke with Gerasimov, Secretary Tillerson has spoken to foreign minister Lavrov. We have had communications at three different levels," Dunford told reporters as he flew back from a NATO Military Committee meeting in Tirana.
He said Russian forces were pursuing IS fighters who had fled across the Euphrates when their jets struck close enough to injure SDF troops in the area.
"We have been engaged at every level to re-establish deconfliction at the Euphrates river. It couldn't be more complex and crowded in that area, and so deconfliction is more difficult right now than it was a few months ago," Dunford said.
"We haven't resolved all the issues right now. We'll get through that," he added.
Regime troops are engaged in an offensive against the provincial capital, Deir Ezzor city, while SDF fighters are battling the jihadists further east across the Euphrates.
The SDF and Russian-backed Syrian government forces are conducting parallel but separate offensives against IS in Deir Ezzor, the last Syrian province still largely held by the jihadists.
Tillerson is to meet Lavrov in New York on Sunday ahead of this week's UN General Assembly, according to Tillerson's updated public schedule.