Twelve Afghans died Sunday when two rockets fired at insurgents missed their target and struck a house during the second day of NATO's most ambitious effort yet to break the militants' grip on the country's dangerous south. In the town of Marjah, the focal point of the operation, Marines and Afghan troops battled through sniper fire and an afternoon sandstorm Sunday that cut down their visibility to a few feet, AP reported. They tried to advance through the town, clearing houses one-by-one of explosives. But gunfire forced them numerous times to take cover in drab mudbrick compounds that they hadn't yet cleared of booby-traps. To the north, U.S. Army troops fought skirmishes with Taliban fighters, calling in a Cobra attack helicopter against the insurgents. NATO said two rockets from a High Mobility Artillery Rocket System were aimed at insurgents firing on Afghan and NATO forces, but struck 1,000 feet (300 meters) off their intended target. The rockets struck a house, killing 12 civilians, NATO said. The civilian deaths were a blow to NATO and the Afghan government's attempts to win the allegiance of Afghans and get them to turn away from the insurgents. «We deeply regret this tragic loss of life,» said Gen. Stanley McChrystal, the top commander in Afghanistan. «The current operation in central Helmand is aimed at restoring security and stability to this vital area of Afghanistan. It's regrettable that in the course of our joint efforts, innocent lives were lost.» McChrystal said he had apologized to Afghan President Hamid Karzai for the accident and had suspended use of the rocket system until the incident can be reviewed. Karzai ordered an investigation into who fired the rocket. Before the offensive began on Saturday, Karzai pleaded with Afghan and foreign military leaders to be «seriously careful for the safety of civilians.» NATO said one service member was killed in a bombing in southern Afghanistan but it was unrelated to the offensive in Marjah. Coalition forces have so far had two casualties in the offensive _ an American and a Briton. Afghan officials said at least 27 insurgents have been killed in the operation.