The United States and Russia established a hotline in 2015 in which a US colonel in Qatar and a Russian colonel in Syria work to "deconflict" operations and prevent aerial collisions.
The jumbled mess of competing forces in and around al Bab has increased the likelihood of mistakes or miscalculations.
Secretary of Defense Gen. James Mattis shakes hands with U.S. Army Lt. Gen. Stephen Townsend, Combined Joint Task Force-Operation Inherent Resolve Commander, during Mattis' inaugural visit to Baghdad, Iraq in support of Operation Inherent Resolve on February 20.
However, there were some casualties among the Syrian Arab Coalition, the commander added. American advisory forces were fewer than five miles away, but were unharmed.
The incident on Tuesday marked the second time in a month that Russian aircraft mistakenly attacked friendly forces.
In this photo taken on Saturday, Oct. 3, 2015, Russian SU-24M jet fighter armed with laser guided bombs takes off from a runaway at Hmeimim airbase in Syria. "The ISIS fighters withdrew, and the Syrian Arab Coalition fighters advanced into those villages", the general explained. "Everybody should keep their sights focused on ISIS. and not fighting deliberately or accidentally one another".
The incident happened southeast of the city of al-Bab, which Townsend said has been fully "liberated" by Turkish forces. The SDF is made up of about 23,000 Arab and 27,000 Kurdish fighters, who are expected to be at the forefront of the push on Raqqa later this year.
"Last week, we saw Turk and Turk proxy forces fighters converge with Syrian regime and Syrian proxy fighters, ISIS being in the mix there".
Al-Bab was liberated last week from the terrorist group by Turkish forces - backed by some 50 USA airstrikes - after months of fighting.
And America is trying to support the Kurds while not alienating ally Turkey. He also said that Turkey's forces are moving toward the Euphrates River, where the SDF is based.
"I believe [the Russians and Syrians] thought [those villages] were held by [the Islamic State group], but who was actually on the ground were some of our Syrian Arab Coalition force", Townsend told reporters at the Pentagon in a briefing from his headquarters in Baghdad. "And we used that mechanism and it worked". They had seen ISIS move out of the area in advance of the regime and the Turks' advance.
Currently, about 500 US troops are in Syria but are not involved in direct combat and are away from the front lines of the fighting.
"This is something that goes on daily in the air", Townsend said of deconfliction.