The guards were killed in Saravan, near Iran’s border with Pakistan, the official IRNA news agency said.
Earlier, the judiciary’s Mizan Online website had quoted local prosecutor Mehdi Shamsabadi as saying that six border guards were killed but later the number was revised down to five.
Sunday’s attack was carried out by “a terrorist group that was seeking to infiltrate the country” but whose members “fled the scene after suffering injuries”, IRNA said.
Iran foreign ministry spokesman Nasser Kanani condemned the attack saying Iran expects Pakistan to “crack down on terrorist groups” and “try to improve the security of the common borders”.
“Certainly, the aim of these terrorist groups is to disrupt the security of the common borders and the security of the people living on the borders of the two countries,” he added.
Poverty-stricken Sistan-Baluchistan, which also borders Afghanistan, is a flashpoint for clashes with drug smuggling gangs as well as with rebels from the Baluchi minority and with Sunni Muslim extremist groups.
The attack was one of the deadliest in the province in recent months.
On March 11, two policemen were shot dead during clashes with “criminals” in the same region, IRNA reported at the time.