Iranian demonstrators threaten revenge during a rally in front of the Saudi Embassy in Tehran on Sunday to protest the execution by Saudi Arabia last week of Sheikh Nimr al-Nimr.

Iran has expressed “regret” over two attacks on Saudi Arabian diplomatic missions and says it will spare no effort in arresting and prosecuting those responsible.

Iran’s UN envoy Gholamali Khoshroo said in a letter to UN Secretary-General Ban Ki-moon Monday that the Islamic Republic “will take necessary measures to prevent the occurrence of similar incidents in the future.”

Saudi Arabia severed ties with Iran on Sunday after protesters attacked its embassy in Tehran and consulate in Mashad. Iranian fanatics turned violent to protest Saudi Arabia’s execution of a prominent opposition Shiite cleric over the weekend, which predominantly Shiite Iran has denounced.

Khoshroo said more than 40 protesters at the embassy have been arrested and handed over to judicial authorities and that investigators are seeking other possible perpetrators.

In the letter, obtained by The Associated Press, he said Iran supports the Vienna conventions on the protection of diplomats and diplomatic property. Khoshroo asked that the letter be circulated to all 193 UN member nations.

UN Secretary-General Ban Ki-moon has urged Saudi Arabia and Iran to support peace efforts in Syria and Yemen and avoid escalating tensions.

UN spokesman Stephane Dujarric says Ban delivered this message in phone calls to Iranian Foreign Minister Mohammad Javad Zarif on Sunday and to Saudi Foreign Minister Adel Al-Jubeir on Monday.

Dujarric said the UN chief called for all political and religious leaders in the region “to avoid adding fuel to the fire.”

The United States also urged Saudi Arabia and Iran to not let their diplomatic spat derail talks to end Syria’s conflict.

White House spokesman Josh Earnest says the US is concerned about the situation and wants the two regional powers to show restraint.

Earnest says it’s in the interests of Iran and Saudi Arabia to continue working toward a political solution for Syria. Both countries have been participating in the talks in Vienna.

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