BEIRUT (AP) — Russian fighter jets have “harassed” American drones over Syria for the third day in a row this week, the U.S. military said.

Tension between Russian and U.S. troops is not uncommon in Syria as both countries conduct patrols on the ground as well as overflights. Syria’s 12-year conflict has left half a million people dead and over 1 million wounded.

The U.S. military said in a statement that Friday’s encounter lasted for about two hours during which three MQ-9 drones were “once again harassed” by Russian fighter aircraft while flying over Syria.

“Russian aircraft flew 18 unprofessional close passes that caused the MQ-9s to react to avoid unsafe situations,” Lt. Gen. Alex Grynkewich, head of U.S. Air Forces Central Command, said in a statement.

Rear Adm. Oleg Gurinov, head of the Russian Reconciliation Center for Syria, said earlier this week that the Russian and Syrian militaries have started a six-day joint training that ends Monday.

Gurinov added in comments carried by Syrian state media earlier this week that Moscow is concerned about the flights of drones by the U.S.-led coalition over northern Syria, calling them “systematic violations of protocols” designed to avoid clashes between the two militaries.

The first friction occurred on Wednesday morning when Russian military aircraft “engaged in unsafe and unprofessional behavior” as three U.S. MQ-9 drones were conducting a mission against the Islamic State group, the U.S. military said. On Thursday, the U.S. military said Russian fighter aircraft flew “incredibly unsafe and unprofessionally” against both French and U.S. aircraft over Syria.

The U.S. and France are part of an international coalition fighting IS that once controlled largest parts of Syria and Iraq where the extremists declared a caliphate. Despite IS defeat in Iraq in 2017 and in Syria less than two years later, the extremists still carry out deadly attack in both countries.

On Friday, a drone attack by the U.S.-led coalition killed a man in northern Syria who was riding a motorcycle. The Syrian Observatory for Human Rights, an opposition war monitor, said the man was an IS militant.

Russia joined Syria’s conflict in September 2015 and has since helped tip the balance of power in favor of Syrian President Bashar Assad’s forces. Russian warplanes still carry out attacks against the last major rebel stronghold in Syria’s northwest.

On any given day there are at least 900 U.S. forces in Syria, along with an undisclosed number of contractors, who partner with the Kurdish-led Syrian Democratic Forces.

“We continue to encourage Russia to return to the established norms of a professional Air Force so we can all return our focus to ensuring the enduring defeat of ISIS,” Grynkewich said, using a term to refer to IS.

Gurinov, the Russian officer, warned that the increase of “uncoordinated flights” for the coalition’s drones leads to escalation and “Russia is not responsible for the safety of these flights.”