Russian warplanes bombed a Syrian Arab Coalition force trained and advised by US troops in northeastern Syria on Tuesday and inflicted casualties, the top USA commander for Iraq and Syria said Wednesday.
Russia's Defense Ministry denied the claim, state-sponsored Sputnik News said.
Gen. Stephen Townsend said a Russian aircraft hit Syrian Arab Coalition fighters, thinking they were bombing Daesh positions in villages near al-Bab, north-east of Aleppo on Tuesday.
Townsend said USA forces were "four or five kilometers away" advising "at command echelons a little bit farther back" when the strikes occurred on the villages.
The Syrian Arab force took casualties, but the Russian planes peeled off once their American counterparts used a long-established hotline to warn them they were bombing friendly forces.
Russian Federation deployed aircrafts to Syria in late 2015 to aid President Bashar al-Assad in the fight against ISIS, while USA troops support the Syrian opposition forces, and the two sides set up a communication channel to avoid getting into each other's air space.
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"The ISIS fighters withdrew, and the Syrian Arab Coalition fighters advanced into those villages", Townsend said of the fighters who were targeted, using a common acronym for the Islamic State.
Earlier Wednesday, a United States commander accused Russian and Syrian jets of bombing US-backed fighters in Syria.
The general sought to tamp down tensions, saying that "the coalition is encouraged" by the progress against ISIS by the Turkish military and their proxies. In this strategy the United States offers the resources of the USA military - including airpower, intelligence, training and arms - while locals carry out the front-line fighting.
The U.S. troops observed the airstrikes and notified higher command to contact the Russians under an arrangement to "deconflict" U.S. and Russian operations in Syria, he said in a briefing from Baghdad to the Pentagon.
"There were US forces in the area", he added.
Ankara does not want armed Kurds, whom they consider terrorists, to take part in the fight for Raqqa, but USA military leaders have said that they are the most reliable fighting force on the ground in Syria.