Twenty-five Iraqi tourists were killed and 16 injured when their bus plunged off a cliff in Iran's northern mountains on Tuesday, news agencies reported.A brake defect caused the driver to lose control, sending the bus crashing 200 meters (650 feet) down into a river in the Alborz mountain range, Iran's Mehr agency said.Authorities were still searching for one body, which might have been carried away by the river, a provincial crisis response official was quoted as saying.Iran's IRNA news agency said the Iraqis were on their way to a Caspian Sea resort, while Mehr said they could have been pilgrims.The crash was the second of its kind this year in the Mazanderan region, known for its dramatic scenery and ski resorts. In April, another bus plunged off a cliff after a mechanical failure, killing 12 people and injuring 28, Mehr said.More than 23,000 people were killed on Iran's roads in 2010, according to the World Health Organization, which observed poor enforcement of traffic laws and low safety standards on locally manufactured vehicles which dominate the domestic market.