A bomb exploded in the southern Iranian city of Ahwaz, hours after two men were hanged for an attack last year, according to Iranian reports.
The percussion bomb shattered the windows of a building in the Kianpars area of the city on Thursday evening, but no casualties were reported.
The attack is the latest in a series to hit the restive Khuzestan Province, at the heart of Iran's oil industry.
Eight people died in bomb attacks on a government office and bank a month ago.
Iran has accused British forces stationed just across the Iran-Iraq border of co-operating with ethnic Arab separatist groups who said they were behind the blasts. The UK has denied any involvement.
The deputy governor-general for security of Khuzestan Province, Mohsen Farokhinejad, confirmed Thursday's blast had been caused by a percussion bomb.
Percussion bombs usually produce a loud bang, but do not cause damage.
"The blast was in Kianpars Ninths Avenue and broke the windows of some nearby buildings," he told the ISNA news agency.
Mr Farokhinejad said no arrests had been made in connection with the attack.
On Thursday, two men found guilty of bomb attacks that killed six people in Ahwaz in October 2005 were publicly hanged at the place where the bombings occurred.
One of those executed said he had contacted Arab separatist groups based in Canada and Britain after reading their websites.
In November, protests erupted in Ahwaz after ethnic Arabs accused Iran's Persian majority of discrimination.