An official in northern Vietnam says rock slides have killed nine people in two separate accidents at quarrying operations.Local official Nguyen Xuan Truong says four workers died at the scene following the first accident Monday in the northern port city of Hai Phong. Two others died in a hospital, and four workers remain hospitalised.The cause remains unclear, but falling rocks buried workers below and crushed them, he said.Truong says a rock slide at a nearby quarry Tuesday killed three other workers.The quarry operations produce limestone for cement factories and construction.