A "sound bomb" exploded in the restive southern Iran city of Ahvaz before dawn on Sunday but did not cause any casualties, the official IRNA news agency said.
"This bomb exploded at 2:45 am (2215 GMT Saturday) near a bridge located close to Ali-ebn Mahziar Ahvazi," a Shiite shrine, deputy governor of Ahvaz Reza Nejati said.
Nejati said there were no casualties or damage caused by what he described as a "sound bomb" but the Mehr news agency said it damaged one of the pillars of the bridge, which carries a railway track.
Ahvaz, which has a sizeable Arab minority, has been hit by deadly bombings in recent years.
The city - the capital of Iran's oil-rich Khuzestan province, which borders Iraq and the Gulf - was also rocked by ethnic violence in April 2005.
Iran in the past has blamed US and British agents in neighbouring Iraq and Afghanistan for launching attacks on its border provinces with significant ethnic minority populations.