(CNN) -- The U.S. military said it is investigating claims from Syria that U.S. helicopters based in Iraq killed eight people and wounded another Sunday in an attack inside Syria's territory Sunday. A U.S. army Apache helicopter flies over southern Baghdad, Iraq. Syria's state news agency SANA said four U.S. helicopters crossed the border and struck a farm about 8 kilometers (5 miles) inside Syria before returning to Iraqi airspace. The raid occurred about 4:45 p.m. (1345 GMT). The helicopters hit a civilian building under construction on the farm, killing a father and his four sons, a married couple and another man, SANA said.