At least 13 people were killed after an oil tanker collided with a passenger bus in Iran's Northwestern city of Sanandaj, Kurdistan province. Seven people including four bus passengers and three people from passing vehicles were also injured in the incident, according to Iranian media. The bus was carrying passengers from Sanandaj to the capital city of Tehran. The incident occurred in mid-night when the tanker carrying fuel oil, failed to stop due to technical failure of brakes and collid with bus, which led to explosion. Sanandaj police officer, Colonel Ramezan Naderi said the passenger bus had stopped out of bus terminal when the crash happened. The officials are still probing into the incident to find out about its causes, he added. The death toll of road incidents over the past several years has been in increase in Iran. The poor quality of vehicles and the failure of drivers to observe traffic regulations are believed to be among the main reasons of the increasing road accidents in Iran.