Iran has accused Pakistan of having links to bombers who killed 42 people - including six commanders of the elite Revolutionary Guards - in a suicide attack near the Pakistani border.
The attack is being blamed on a Sunni Muslim resistance group called Jundullah, which Iran has previously accused of terrorist activities.
Pakistan, which has backed Sunni Muslim groups in the past, has condemned the attack and is denying any involvement; but Iranian president Mahmoud Ahmadinejad is calling on Islamabad to hand over those involved.
State media say that one or more bombers targeted a meeting of the guards with tribal leaders in the province of Sistan-Baluchestan bordering Pakistan.
The province is made up mainly of the Baluchi ethnic group, who belong to the Sunni minority of Shia-ruled Iran.
Pakistan has in the past backed Sunni militant groups, particularly in Afghanistan in the 1980s, when they were resisting Soviet occupation.