Iranian state TV said a bus carrying pilgrims on their way to holy sites in Iraq has crashed into a warehouse in western Iran, killing 28 people.
The report said nine others were seriously injured.
Monday's accident occurred near Ilam, 710 kilometers southwest of the capital, Teheran.
The TV quoted a police official as saying high speed and mechanical problems were the main causes of the accident.
Iran has one of the worst road safety records in the world. According to official statistics, more than 26,000 people die in road accidents in Iran each year due to unsafe vehicles, disregard for traffic rules and inadequate emergency services.