Russian and Syrian aircraft bombed positions held by the U.S.-backed Syrian Arab Coalition near the Syrian town of al Bab, inflicting casualties, the top U.S. commander in Iraq said on Wednesday, but Moscow denied that had happened.
"Yesterday, we had some Russian aircraft and (Syrian) regime aircraft bomb some villages that I believe they thought were held by ISIS, yet they were actually - on the ground - were some of our Syrian Arab coalition forces," Army Lieutenant General Stephen Townsend told a Pentagon news briefing, using an acronym for the Islamic State militant group.
Russia's RIA news agency cited the Russian defense ministry as denying Russian or Syrian aircraft had launched any strikes on positions held by the U.S.-backed Syrian Arab Coalition.
The U.S. military provided Russia with exact coordinates of Syrian opposition forces it supports and Russia took that information into account, RIA cited the Russian defense ministry as saying.
"Not one strike was launched by Russian or Syrian aviation on the areas given by the American side," the Russian defense ministry said, according to RIA.
The villages where the Tuesday strikes took place were close to al Bab and about 15 or 20 km (10 to 12 miles) from Manbij city, Townsend said.
U.S. forces in the area, four or five km (2-1/2 to three miles) away, observed the strikes and the U.S. military called their Russian counterparts through an emergency line, after which the bombing stopped, Townsend said.
"Some quick calls were made through our deconfliction channels and the Russians acknowledged and stopped bombing there," he added.
The United States and Russia have a channel for avoiding each other in the crowded airspace over Syria.
In 2015, they agreed to create a ground communication link and outline steps their pilots could take to avoid an inadvertent clash.
Some U.S. military commanders have advocated creating an additional channel involving more senior officers given the close proximity of fighting in Syria.