By Hossein Jaseb and Fredrik Dahl
The renewed violence occurred ahead of a presidential election on June 12 in which conservative President Mahmoud Ahmadinejad, who often rails against foreign threats to Iran's security, is pitted against moderates seeking detente with the West.
Two passengers were wounded in the attack on the bus west of the city of Zahedan, a police chief said. A suicide bomber killed 25 people in a Shi'ite mosque in Zahedan last Thursday and another six were killed in unrest there on Sunday.
Zahedan is the capital of Sistan-Baluchestan province, where most people are minority Sunni Muslims and ethnic Baluchis.
Close to Pakistan and Afghanistan, the region has seen frequent clashes between security forces and heavily armed drug smugglers, as well intermittent attacks by Sunni Baluchi rebels.
Deputy police commander Ahmad-Reza Radan said rebels set fire to a vehicle carrying natural gas on the same road near Zahedan but nobody was killed or wounded, the official IRNA news agency said, but it was not clear whether it was a separate incident to the attack on the bus.
STREET DISTURBANCES
Judiciary spokesman Alireza Jamshidi said between 20 and 30 people had been detained over street disturbances in Zahedan in which he said six people were killed this week. A police official said many more had been taken into custody.
State television said clashes between backers and opponents of a Sunni cleric broke out in Zahedan on Sunday, a day after three men convicted of involvement in the mosque bombing were executed in public.
Sectarian violence is relatively rare in the officially Shi'ite Muslim country, whose leaders reject allegations by Western rights groups that the country discriminates against ethnic and religious minorities.
Officials have blamed Thursday's mosque bombing in Zahedan and a separate incident two days later when a bomb was found on a plane on Iran's foes. One said they wanted to "create a security-threat environment" before the presidential election.
A Sunni opposition group named Jundollah (God's Soldiers), which Iran says is part of the Islamist al Qaeda network and backed by the United States, said it was behind the mosque bombing, Saudi-owned Al Arabiya television reported last week.
Jamshidi said two people were held over Saturday's plane incident, in which Iranian media said security personnel had defused a homemade bomb found on a domestic flight to Tehran.
Separately, 11 people were detained in the northwestern, mainly ethnic Azeri city of Tabriz on accusations that included "gathering with the intention of committing crimes against national security and keeping firearms", Jamshidi said. He gave no details. (Editing by Jon Hemming)
