Saudi Arabia's top religious authority said Iran's leaders were not Muslims, drawing a rebuke from Tehran in an unusually harsh exchange between the regional rivals over the running of the annual hajj pilgrimage.
The war of words on the eve of the mass pilgrimage will deepen a long-running rift between the Sunni kingdom and the Shi'ite revolutionary power. They back opposing sides in Syria's civil war and a list of other conflicts across the Middle East.
Iranian Supreme Leader Ayatollah Ali Khamenei, in a message published on Monday, criticized Saudi Arabia over how it runs the hajj after a crush last year killed hundreds of pilgrims. He said Saudi authorities had "murdered" some of them, describing Saudi rulers as godless and irreligious.
Responding to a question by Saudi newspaper Makkah, Saudi Arabia's Grand Mufti Sheikh Abdulaziz Al al-Sheikh said he was not surprised at Khamenei's comments.
"We have to understand that they are not Muslims... Their main enemies are the followers of Sunnah [Sunnis]," Al al-Sheikh was quoted as saying, remarks republished by the Arab News.
He described Iranian leaders as sons of "magus," a reference to Zoroastrianism, the dominant belief in Persia until the Muslim Arab invasion of the region that is now Iran 13 centuries ago.
Al al-Sheikh's remarks drew an acerbic retort from Iran's Foreign Minister, Mohammad Javad Zarif, who said they were evidence of bigotry among Saudi leaders.
"Indeed; no resemblance between Islam of Iranians & most Muslims & bigoted extremism that Wahhabi top cleric & Saudi terror masters preach," Zarif wrote on his Twitter account.
Saudi authorities normally seek to avoid public discussion of whether Shi'ites are Muslims, but implicitly recognize them as such by welcoming them to the hajj, and by accepting Iranian visits to the Saudi-based Organization of Islamic Cooperation.
Tensions between the two countries have been rising since Riyadh cut ties with Tehran in January following the storming of its embassy in Tehran, itself a response to the Saudi execution of dissident Shi'ite cleric Nimr al-Nimr.
Custodian of Islam's most revered places in Mecca and Medina, Saudi Arabia stakes its reputation on organizing hajj, one of the five pillars of Islam which every able-bodied Muslim who can afford to is obliged to undertake at least once.
Riyadh said 769 pilgrims were killed in the 2015 disaster, the highest hajj death toll since a crush in 1990. Counts of fatalities by countries who repatriated bodies showed that more than 2,000 people may have died, more than 400 of them Iranians.
Iran blamed the 2015 disaster on organizers' incompetence. Pilgrims from Iran will be unable to attend hajj, which officially starts on Sept. 11, this year after talks between the two countries on arrangements broke down in May.
The split between Islam's main sects dates to a dispute among Muslims over who would rule their community after the death of the Prophet Mohammad, and Shi'ites still regard his descendants as a line of imams blessed with divine guidance.
Today such disagreements over history remain emotive points of tension between the sects, but they are also divided over day-to-day issues including differing interpretations of Islamic law and the role and organization of the clergy.
In the Wahhabi teaching of Sunni Islam followed by the Saudi clergy and government, Shi'ite doctrine about imams is seen as incompatible with the concept of a monotheistic God.