The ambush took place in the restive southeastern province of Sistan-Baluchestan, where Sunni militants have carried out repeated attacks against the security forces of mainly Shiite Iran.
"Terrorists carried out an ambush against a military patrol and, after killing three Revolutionary Guards, fled into nearby Pakistan," the province's deputy governor Ali Asghar Mirshekari told the official IRNA news agency.
A military official cited by the Fars news agency said that the rebel attack took place in the Saravan region on Sunday afternoon and that the dead included a colonel he identified as Akbar Abdollahnejad.
In October, three security forces members were killed in an attack blamed on rebels in Saravan.
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An Iranian soldier was also killed in a September attack on a border post there.
Sistan-Baluchistan province has a large Sunni community and has been plagued by violence involving Sunni extremists and drug smugglers.
Iran accuses members of the Sunni militant group Jaish-ul Adl (Army of Justice) of carrying out frequent raids.
The group captured five Iranian troops in February, four of whom were released in April. The fifth soldier is presumed dead but his fate remains officially unknown.