A US MQ-9 Reaper drone reportedly took out a Soviet-vintage T-72 tank in eastern Syria on Saturday. Military.com says the drone attack was carried out in “self-defense” after forces loyal to President Bashar al-Assad fired at US advisers and allied Syrian fighters.

Lieutenant-General Jeffrey Harrigian, chief of the US Air Force’s Central Command, noted on Tuesday that the battlefield in Syria was becoming increasingly complicated as more contending military units move into the area.

Harrigian declined to speculate on the exact identity of the forces operating the tank – Russian forces or those belonging to Assad.

200 Russian mercenaries killed?

Concern has mounted in recent weeks that US forces and their allies might come into direct contact with Russian troops amid an increasingly confused military situation in Syria.

The drone incident follows an unconfirmed Bloomberg report that US forces killed more than 200 Russian mercenaries in an attack on a US-held military base and refinery in Syria last week. The mercenaries were said to be mostly Russian nationals fighting under contract for Assad.

“More than 200 contract soldiers, mostly Russians fighting on behalf of Syrian leader Bashar al-Assad, died in a failed attack on a base held by US and mainly Kurdish forces in the oil-rich Deir Ezzor region, two of the Russians said. The US official put the death toll in the fighting at about 100, with 200 to 300 injured, but was unable to say how many were Russians,” Bloomberg reported.