Kabul, Dec 6 : Seven civilians were killed and six others injured on Tuesday as a roadside bomb struck a bus of government employees in Mazar-i-Sharif, the capital of Afghanistan's Balkh province, a police spokesman said. It was a roadside bomb that struck the bus of the employees of Petroleum Directorate of Hiratan border town at 7.30 a.m. in Police District 3 of Mazar-i-Sharif city, leaving seven dead and six others injured," Xinhua news agency quoted the spokesman as saying. The employees were going to their workplace when the incident occurred, the official said. Without providing more details, the official said that an investigation had been initiated into the incident. Meanwhile, witnesses have said that the number of casualties could be higher than reported. No group or individual has claimed responsibility for the blast. /IANS