A suicide attack on Wednesday on a Revolutionary Guards bus in southeastern Iran killed at least 20 people, the official news agency IRNA reported.
"The suicide attack on an Islamic Revolutionary Guards Corps personnel bus happened on the Khash-Zahedan road," IRNA said.
"This terrorist act happened a short time ago and according to reports at least 20 have been martyred and 20 have been wounded," an informed source told IRNA.
A picture released by the Fars news agency showed a hulk of twisted metal at the scene.
The Guards issued a statement confirming the attack. It said the troops were returning from the border.
"In this suicide attack a car filled with explosives blew up besides a bus carrying a unit of the Guards ground forces causing the martyrdom and wounding of a number of the protectors of our Islamic homeland's border."
The attack took place in the volatile southeastern province of Sistan-Baluchistan which has a large, mainly Sunni Muslim ethnic Baluchi community which straddles the border with Pakistan.
On January 29 three members of an Iranian bomb squad sent to the scene of an explosion in Zahedan, the capital of Sistan Baluchistan province, were wounded when a second device blew up as they were trying to defuse it, police said at the time.
And in early December last year two people were killed and around 40 others wounded in the port city of Chabahar, also in Sistan Baluchistan, in an attack which Foreign Minister Mohammad Javad Zarif at the time blamed on "foreign-backed terrorists" -- a reference to Sunni Muslim extremists.
The bloodiest attack in recent times to have hit Iran took place in September when assailants killed 24 people at a military parade in the southwestern city of Ahvaz.
In July at least 10 members of the Revolutionary Guards were killed when insurgents attacked one of their bases along the border with Iraq.