Russian warplanes intercepted US drones over Syria for the third day in a row this week, the US military said.
Tension between Russian and US forces is not uncommon in Syria as both countries conduct ground patrols as well as overflights.
In the 12-year conflict in Syria, half a million people have died and more than a million have been wounded.
The US military said Friday's "encounter" lasted about two hours, during which Russian fighter jets "once again harassed" three MK-9 drones over Syria.
"Russian aircraft flew 18 times close to MK-9 drones that avoided the precarious situation," the US military said.
Russian officer Oleg Gurinov, head of the Russian Center for Reconciliation in Syria, said this Sunday that the Russian and Syrian armies had begun six days of joint training that ends on Monday.
Gurinov said Moscow was concerned about drone flights by the US-led coalition over northern Syria, saying the drones "systematically violated protocols" to avoid incidents.
The US and France are part of an international coalition fighting the Islamic State, which once controlled large parts of Syria and Iraq where the extremists have declared a caliphate.
Despite the defeat of Islamists in Iraq in 2017 and in Syria less than two years later, extremists continue to carry out deadly attacks in both countries.
Russia joined the conflict in Syria in September 2015 and helped defeat the forces of Syrian President Bashar al-Assad.
Russian warplanes continue to attack the last major rebel stronghold in northwestern Syria.
There are at least 900 US troops in Syria partnering with the Kurdish-led Syrian Democratic Forces.
Gurinov warned that the increase in "uncoordinated flights" of drones by the US-led coalition leads to escalation and that "Russia is not responsible for the safety of those flights."
Bonus video:
