Saudi Arabia has announced that it has severed diplomatic ties with Iran.
This comes amid a row over the Saudi execution of a prominent Shia Muslim cleric. Sheikh Nimr al-Nimr and 46 others were executed on Saturday after being convicted of terror-related offences.
Saudi Foreign Minister Adel al-Jubeir who spoke after demonstrators had stormed the Saudi embassy in Tehran said that all Iranian diplomats must leave Saudi Arabia within 48 hours.
Saudi Arabia was recalling its diplomats from Tehran, he said.
Mr Jubeir said Saudi Arabia would not let Iran undermine its security, accusing it of having "distributed weapons and planted terrorist cells in the region".
"Iran's history is full of negative interference and hostility in Arab issues, and it is always accompanied by destruction," he told a news conference.
US state department spokesman John Kirby said: "We will continue to urge leaders across the region to take affirmative steps to calm tensions".
"We believe that diplomatic engagement and direct conversations remain essential," he said.
Earlier, Iran's Supreme Leader Ayatollah Ali Khamenei warned that the Sunni Muslim kingdom would face "divine revenge" for the execution - an act which also angered Shia Muslims elsewhere in the Middle East.
Ayatollah Khamenei called Sheikh Nimr a "martyr" who had acted peacefully.
Protesters stormed the Saudi embassy in Tehran late on Saturday, setting fire to the building before being driven back by police. The Saudi foreign ministry said none of its diplomats had been harmed in the incident.
Iran is Saudi Arabia's main regional rival - they back opposing sides in the conflicts in Syria and Yemen.
Relations between the countries have been strained over various issues in recent decades, including Iran's nuclear programme and deaths of Iranians at the Hajj pilgrimage in 1987 and again in 2015.