US helicopters attacked a group of extremists preparing to fire a rocket toward the Green Zone, the US military said Wednesday. Apache helicopter crews from the 1st Battalion, 227th Aviation Regiment spotted a team of 10-15 militants setting up a rocket launcher late Monday in Sadr City, the Shi'ite district and stronghold of the Mahdi Army militia, the US said in a statement. The militants tried to get away as the helicopters approached, the statement said. The crews received clearance to fire and engaged a van and two cars in which the extremists were trying to escape, the statement said. Three vehicles and the launcher were destroyed, the statement said.