Iran has warned the U.S., Britain and Pakistan of retaliation after a suicide bombing killed 42 of its elite Revolutionary Guard, including five senior officials.
The commanders were entering a sports complex on Saturday near the Pakistani border when the attack took place.
They were meeting with tribal leaders to discuss Sunni-Shiite cooperation in an outlaw region of armed tribal groups, drug smugglers and Sunni rebels.
Iranian President Mahmoud Ahmadinejad is vowing that the attackers "will soon get the response for their anti-human crimes."
Meanwhile, a spokesman for the U.S. State Department has condemned the attacks and called claims of U.S. involvement "completely false."