Saudi Arabia's top cleric has said Iran's leaders are "not Muslims", a day after Iran's supreme leader denounced the management of the Hajj pilgrimage.
Abdul Aziz Al Sheikh, the grand mufti, said Ayatollah Ali Khamenei's accusations were "not surprising".
"They are the sons of the Magi," he said, referring to Zoroastrianism, a religion that once dominated Iran.
Deep suspicions exist between predominantly Sunni Muslim Saudi Arabia and its mainly Shia Muslim neighbour.
On Monday, Ayatollah Khamenei accused Saudis of "murdering" pilgrims caught up in a stampede at last year's Hajj.
"The heartless and murderous Saudis locked up the injured with the dead in containers - instead of providing medical treatment and helping them or at least quenching their thirst," he said, without providing evidence. "They murdered them."
"The world of Islam, including Muslim governments and peoples, must familiarize themselves with the Saudi rulers and correctly understand their blasphemous, faithless, dependent and materialistic nature. They must not let those rulers escape responsibility for the crimes they have caused throughout the world of Islam," he added.
The ayatollah made the allegation on the anniversary of the stampede, which killed at least 2,426 people, including 464 Iranians, according to an unofficial count.
The Saudi authorities, who say 769 died, have made few details of their investigation into the stampede public but previously rejected criticism.
Sheikh Al Sheikh was dismissive when asked by the Mecca newspaper about Ayatollah Khamenei's comments.
"We must understand these are not Muslims," he was quoted as saying. "They are the son of the Magi and their hostility towards Muslims is an old one, especially with the People of the Tradition [Sunnis]."
The newspaper said Sheikh Al Sheikh's remarks were directed towards Ayatollah Khamenei and the Iranian leadership.
Animosity between Sunnis - who make up an estimated 85-90% of Saudi Arabia's population - and Shia - about 90-95% of Iran's population - goes back to a 7th Century schism.
Hardline Sunnis - including many adherents of Wahhabism, the austere form of Islam practised by the Saudi ruling family and religious establishment - often describe Shia as "rejectionists" who have strayed from the true faith.
The war of words between the clerics also comes nine months after Saudi Arabia severed diplomatic ties with Iran in response to attacks on Saudi diplomatic compounds in Iran by people angered by the kingdom's execution of a prominent Shia cleric convicted of terrorism offences.
Correction 10 October 2016: This article has been amended to make clear that Sheikh Al Sheikh was referring to the Iranian leadership rather than the people of Iran, according to the Mecca newspaper report