Russian fighter jets have “harassed” American drones over Syria for the third day in a row this week, the US military said.

The US 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 US 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 on Monday.

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

US, Russia tensions

Tension between Russian and US troops is not uncommon in Syria as both countries conduct patrols on the ground as well as overflights.

The first friction occurred on Wednesday morning when Russian military aircraft “engaged in unsafe and unprofessional behaviour” as three US MQ-9 drones were conducting a mission against Daesh terror group, the US military said.

On Thursday, the USmilitary said Russian fighter aircraft flew “incredibly unsafe and unprofessionally” against both French and US aircraft over Syria.

On any given day there are at least 900 US forces in Syria, along with an undisclosed number of contractors, who partner with the PKK/YPG terror group.

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.”

Russia joined Syria’s conflict in September 2015 and has since helped tip the balance of power in favour of the Syrian regime forces under Bashar al Assad.