Fire fueled by natural gas swept through three buildings, forced at least 10 families from their fashionable Russian Hill apartments and caused more than $1 million in damage, San Francisco fire officials said. There were no serious injuries in the blaze that started in a four-story building and spread to structures on either side, but several firefighters suffered minor injuries and smoke inhalation. Many of the families were being cared for by the Red Cross, which offered temporary housing, food and clothing. The fire caused $350,000 in damage to the initial structure and $750,000 in damage to the adjoining set of flats. Two other buildings were less seriously damaged, Battalion Chief Bill Zerbe said. The cause of the fire was unknown but possibly faulty basement gas meters were being looked at, he added. The fire was fed by natural gas and spread upward through light wells between the buildings.