Pentagon says it's almost like Russia's on a 'mission to protect ISIS leaders' after its fighter jets harassed US Reaper drones that took out a top terrorist

• Russian fighter jets harassed several US military drones operating above Syria on Friday. 
• Those same American MQ-9 Reaper drones later went on to kill an ISIS leader during an operation.
• A Pentagon official said Monday it's almost like Moscow is "on a mission to protect ISIS leaders."

Russia is acting like it's on a "mission to protect" Islamic State leaders by disrupting counter-terrorism operations, a Pentagon official said Monday after Russian fighter jets harassed US military drones that were sent to eliminate a top terrorist just a few hours later.

US officials have slammed Russia's air force in recent days for carrying out aggressive maneuvers around several NATO aircraft operating above Syria on multiple occasions throughout last week. Friday marked three days in a row of what Pentagon leadership has slammed is "reckless" and "unprofessional" behavior by Moscow's pilots.

During the nearly two-hour-long engagement on Friday, an unspecified number of Russian aircraft flew 18 close passes near three Reaper drones, which were forced to "react to avoid unsafe situations," Lt. Gen. Alexus Grynkewich, the commander of US Air Forces Central Command (AFCENT) said at the time. 

Several hours later though, these same Reaper drones carried out a strike in eastern Syria that killed Usamah al-Muhajir, who was an ISIS leader. US Central Command (CENTCOM), which revealed the strike on Sunday, said it was unaware of any civilian deaths but was reviewing reports that a civilian may have been injured.

"We have made it clear that we remain committed to the defeat of ISIS throughout the region," said Gen. Michael "Erik" Kurilla, CENTCOM commander. "ISIS remains a threat, not only to the region but well beyond."

The US has around 900 troops deployed to Syria, where they carry out missions against ISIS targets. Russia, meanwhile, operates in the country in support of the Assad regime's ongoing civil war. But in recent months, Washington has called attention to increasingly dangerous actions by Russian pilots who it says are interfering with America's interests there. The US and its partners saw several of these incidents last week alone. 

Deputy Pentagon Press Secretary Sabrina Singh told reporters on Monday that the US is vocal and clear about its mission in Syria to defeat ISIS, according to a Pentagon official who relayed her comments from a Monday press gaggle.  

"The Russians know that, they know exactly where we operate, and so there is no excuse for Russian forces' continual harassment of our MQ-9s after years of US operations in the area aimed at ensuring the enduring defeat of ISIS," Singh said. "And it is almost as if the Russians are now on a mission to protect ISIS leaders."

"So while I can't speak to other missions, if they have been deterred by Russian actions, what I can say is that CENTCOM was able to successfully execute this mission despite what the Russian forces have been trying to do in the air," she said. 

Last week saw several instances where Russian fighter jets performed "unsafe and unprofessional" actions near NATO aircraft operating above Syria, according to US Air Force officials. On Wednesday, Russian Su-35s dropped parachute flares and engaged afterburners in front of three Reaper drones.

The following day, Russian Su-34 and Su-35 fighter jets dropped flares and flew "dangerously close" to an unspecified number of Reaper drones. In a separate incident also on Thursday, a Su-35 conducted a "non-professional interaction" with two French Dassault Rafale fighters that were in the middle of a security mission. 

"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 after Friday's incident. 

Colin Kahl, the under secretary of defense for policy, told reporters on Friday that the US is concerned about the continuous nature of these incidents and that Washington doesn't want this behavior to be normalized. 

"So, we will continue to highlight when the Russians engage in reckless, unprofessional behavior in the skies, we will highlight for the world so that, when and if an incident happens, people will know who's at fault here," Kahl said.