The Kremlin denied involvement in air strikes on Syria's Eastern Ghouta enclave that have killed at least 250 civilians.
Russia admitted Tuesday that several dozen of its "citizens" had been killed or wounded in recent fighting in Syria.
Russia acknowledged Thursday that five of its citizens may have been killed by a U.S. strike in Syria.
U.S. officials said they had no information on such casualties, and the Kremlin did not confirm any Russian deaths.
A U.S. MQ-9 Reaper drone took out a Soviet-made T-72 tank in eastern Syria on Saturday in a "self-defense" strike..
Israel's air strikes marked its most serious engagement in Syria since fighting there began in 2011.
Keeping U.S. forces in areas claimed by the Assad regime inherently increases the probability of more clashes.
An Israeli military spokesman reported that the F-16 pilots are "safe."
Air Force Col. John Thomas said that El Shafee Elsheikh and Alexanda Amon Kotey were captured in January in eastern Syria.
The U.S. sent up the F-22A Raptor advanced stealth fighter, along with drones, to watch as the three-hour battle began.