"Armed terrorists entered Iran from Pakistan and clashed with border guards, killing eight soldiers before fleeing back to Pakistan," Ali Asghar Mirshekari, deputy governor of Sistan-Baluchistan province, told the news agency.
The southeastern province has a large Sunni community and has seen repeated attacks by militants on the security forces of mainly Shiite Iran.
But the clash in the Negur district was the deadliest since October 2013 when 14 border guards were killed in an attack claimed by Sunni militant group Jaish-ul Adl (Army of Justice).
The same group captured five Iranian troops in February last year. Four of them were released last April. The fifth is still missing presumed dead.