The bus was heading from the city of Jaffna to the town of Diyatalawa, the newspaper stated.
"The explosion cannot be ruled out as an act of terrorism as of yet. Nevertheless, a highly explosive substance inside the bus may have caused the explosion. The cause can be ruled out after the government analyst inspects the scene," Sri Lankan army spokesman Brig. Sumith Atapattu was quoted as saying.
According to the spokesman, the number of casualties could be higher, but part of passengers was transferred before the incident to another bus heading to the same destination.