A suicide bomber killed four security personnel outside the Prophets Mosque in Medina, the second-holiest site in Islam, on Monday, the same day as devices exploded outside a Shiite mosque in eastern Saudi Arabia and near the U.S. Consulate in Jeddah.
"There are no more red lines left for terrorists to cross. Sunnis, Shiites will both remain victims unless we stand united as one," Irans Foreign Minister Mohammad Javad Zarif tweeted.
Iran, the leading Shiite Muslim power, has been calling for rapprochement with Sunni Muslim Saudi Arabia, after years of rivalry climaxed in a break-down of diplomatic relations in January.
The countries are both fighting the ultra-hardline Sunni fighters of Islamic State - Tehran has sent in troops to confront the movement and other militants in Syrias civil war while Riyadh has been hit by a series of attacks claimed by the jihadists.
"Terrorism does not have any border and does not recognise any nationality. There is no other solution but to create a regional and international united front against it," Iranian foreign ministry spokesman Bahram Qasemi was quoted as saying by state news agency IRNA.
There was no immediate response from Saudi Arabia, which accuses Iran of spreading sectarian strife by backing Shiite militias in Yemen, Iraq and Syria and encouraging discord in the kingdoms largely Shiite east.
Saudi Arabia cut diplomatic relations with Iran in January after Iranian protesters attacked Saudi diplomatic missions in Tehran and Mashhad following Riyadhs execution of a prominent Shiite cleric, Reuters reported.
Their ties have been tested even further by their backing for opposing sides in Syrias war, and by security at annual haj pilgrimage.
More than 2,000 pilgrims, 464 of them Iranian, died in a crush at last years haj in the sacred Saudi city of Mecca.