Russian and Syrian forces "accidentally" bombed US -backed fighters in Syria on Tuesday, killing some Syrian rebels, a top USA general said Wednesday.
"Some quick calls were made through our deconfliction channels and the Russians acknowledged and stopped bombing there".
The Russian ministry has denied Townsend's claim.
The Russian Foreign Ministry said that "on February 28, there were contacts in Syria via the established communication channels between a representative of the air group command of the Russian Aerospace Forces at the Hmeymim airbase and his United States counterpart at the Al Udeid air base (Qatar)".
He referred to the Syrian forces to disclose the number and extent of the casualties.
Townsend said the bombings occurred south and east of al-Bab.
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It's unclear whether President Donald Trump will seek military coordination with Russia in Syria; his predecessor deemed it inappropriate, arguing that the Russians were at cross purposes with the U.S.by acting to prop up the Syrian government.
Townsend, who commands US forces in Iraq and Syria, said these groups are "bumping up against each other" around the city, adding "three armies and an enemy force have all converged within the same grid square, so it's very hard and complicated" to know just who is where.
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. "And we used that mechanism and it worked".
His comments were a diplomatic way to express American alarm over Turkish threats to move from al Bab on to the nearby city of Manbij, where the US -backed Syrian Democratic Forces hold ground.
The need for these so-called "deconfliction" talks are becoming more critical as ISIS increasingly loses ground and American and Russian aircraft are operating in a small area.
Turkey has moved into northern Syria and is battling IS while also trying to keep US-backed Kurdish forces in check.
"But in that event that we bring in any additional troops, we'll work that with our local partners, both here in Iraq and Syria, to make sure that they understand the reasons why we're doing that and to get their buy-in of that".