It's the latest development in a rift that deepened this weekend, after Saudi Arabia executed a leading Shiite cleric. All Saudi diplomatic personnel in Iran have been called home.

Manama ordered Tehran's diplomats in the tiny Gulf state "to leave the kingdom within 48 hours", BNA state news agency said on Twitter.

The United Arab Emirates (UAE) on Monday chose to downgrade its diplomatic relations with Iran to that of charge d' affaires-level and to require a reduction in the number of Iranian diplomats stationed in the UAE.

Al-Azhar, the Cairo-based seat of Sunni Muslim learning, and the Organization of Islamic Cooperation in Saudi Arabia, condemned the attacks on Riyadh's missions and stressed Tehran's obligation to respect the internal affairs of the kingdom.

"What we have seen during the last 24 hours is unprecedented..."

France and Germany on Monday echoed a call by the United States for Saudi Arabia and Iran to talk to each other and reduce tensions, while China said it was concerned about the prospect of conflict in the Middle East intensifying as a result of the row.

The supreme leader of Iran, Ayatollah Ali Khamenei, warned on Saturday that Saudi Arabia would face divine vengeance for the execution of al-Nimr, who was put to death in a mass execution of 47 men accused of terrorism-related offenses. The executions have sparked a wave of protests across the Middle East, and among Shi'ite communities all over the world.

"It was essential to bring Saudi Arabia and Iran together, and there was some progress", said Hussein Ibish, senior resident scholar at the Arab Gulf States Institute in Washington.

With files from the Associated Press.

The execution of Nimr provoked protests among Shi'ites across the region and Iranian protesters stormed the Saudi embassy in Tehran, setting fires and causing damage, prompting Riyadh to cut ties and inflaming an already heated rivalry.

Bahrain frequently accuses Iran of being behind the long-running, low-level insurgency in the country since its majority Shiite population began protests in 2011 against Bahrain's Sunni rulers.

At the White House, spokesman Josh Earnest urged Saudi Arabia and Iran not to let their dispute derail fragile talks aimed at securing a cease-fire and a political transition to end the war in Syria.

Bahraini officials have since accused Iran of training militants and attempting to smuggle arms into the country, which hosts the U.S. Navy's 5th Fleet.

"The diplomatic police are responsible for confronting any aggression against the diplomatic sites of Saudi Arabia and will act according to its duties to maintain public order and restore security to such places", Foreign Ministry spokesman Hossein Jaber Ansari said.