By Pam O'Toole
BBC regional analyst
A shadowy Sunni Muslim group has claimed responsibility for bombing a bus carrying Revolutionary Guards in Iran, killing 11 people.
The attack hit in the majority Sunni city of Zahedan, the capital of Sistan-Baluchestan province, in the mostly Shia country.
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Sistan-Baluchestan, in south-eastern Iran, is one of the poorest and most violent provinces in the country.
Bordering Pakistan and Afghanistan, it is one of the main routes for smuggling drugs from Afghanistan to Europe and the scene of frequent gun battles between drug smugglers and Iranian security forces.
The area has a history of banditry and kidnappings aimed both at civilians and Iranian officials. A number of Iranian soldiers have been ambushed or kidnapped there in the past.
Last year more than 20 people, including government workers and officials, were killed in an attack at a checkpoint in the province.
'Army of God'
But Wednesday's attack on the Revolutionary Guards was said to have been the biggest bomb explosion in the city for some years.
A group called Jundallah - or Army of God - has claimed responsibility. Iranian authorities have blamed a number of violent incidents in recent years on Jundallah, which they describe as a group of bandits.
Local officials have alleged that Jundallah mounts cross-border attacks from Pakistan. After the latest blast, an MP for Zahedan, Hossein Ali Shahriyari, called for the issue to be taken up with Pakistan.
Speaking to the BBC later, Mr Shahriyari also echoed allegations made by some other Iranian officials last year, who maintained that bandit groups in Sistan-Baluchestan had links with British and American intelligence agencies.
But he offered no evidence to back up such allegations.
In the past, Jundallah has denied claims by Iranian officials that it has links with al-Qaeda, radical Wahhabi groups, or foreign intelligence agencies. It says it is fighting for the social and religious rights of Sunni Baluchis and for the Iranian people.
The past two years have seen a rise in violent incidents in a number of regions inhabited by Iran's minorities, amid complaints of government oppression, discrimination or political or economic neglect.
As news was emerging of the Zahedan bombing on Wednesday, Iran's media was carrying news of the hanging of three men convicted of carrying out earlier bombings in Khuzestan province in south-west Iran, home to a substantial ethnic Arab population.
Last year also saw angry demonstrations by ethnic Azeris in Iran's Azerbaijan province, while in 2005 there was a wave of protests in Kurdestan province.
Iran's officials have previously accused foreign intelligence agents of being behind a string of violent incidents in parts of the country with substantial ethnic minorities.