The US military says airstrikes have killed nine Shi'ite extremists who were attacking Iraqi soldiers with rocket-propelled grenades in Baghdad's militia stronghold of Sadr City. Lt. Col. Steve Stover says an American helicopter fired a Hellfire missile that killed three of the militants after they were spotted firing at the Iraqi troops. The military spokesman says another missile destroyed a vehicle, killing six militants trying to flee the scene. The military says extremists attacked a crowded market in Sadr City with 107 mm missiles but no casualties have yet been reported. Sunday's attacks come as Iraqi troops backed by US forces clash with Shi'ite gunmen in fighting that has killed at least 20 people and wounded dozens.