The U.S. military launched an air assault on dozens of sites in Iraq and Syria used by Iranian-backed militias and the Iranian Revolutionary Guard Friday, in the opening salvo of retaliation for the drone strike that killed three U.S. troops in Jordan last weekend.<br />''<br />''The massive barrage of strikes hit more than 85 targets at seven locations, including command and control headquarters, intelligence centres, rockets and missiles, drone and ammunition storage sites and other facilities that were connected to the militias or the IRGC's Quds Force, the Guard's expeditionary unit that handles Tehran's relationship with and arming of regional militias.<br />''<br />''The U.S. strikes appeared to stop short of directly targeting Iran or senior leaders of the Revolutionary Guard Quds Force within its borders, as the U.S. tries to prevent the conflict from escalating even further. Iran has denied it was behind the Jordan attack.<br />''President Joe Biden and other top U.S. leaders have been warning for days that the U.S. would strike back at the militias, and they made it clear that it would not be just one hit, but would be a "tiered response" over time. The officials spoke on condition of anonymity to discuss military operations not yet made public.<br />''<br />''At least 13 Iranian-backed fighters have been killed in strikes in eastern Syria, the Syrian Observatory for Human Rights has said. The airstrikes destroyed 17 positions sheltering Iranian militias in Al-Mayadeen and Al-Bokamal near the border between Syria and Iraq, in addition to airstrikes targeting positions near Deir ez-Zor city, it said. Syrian state media said earlier that a number of sites on Syria's desert areas and the Syrian and Iraqi border had resulted in a number of casualties and injuries.<br />''<br />''Six Iranian-backed militia members have been killed and four others injured as a result of airstrikes on Deir ez-Zor in eastern Syria, according to the Syrian Observatory for Human Rights. At least three of those killed were of non-Syrian nationality, the group said.<br />