Bahrain, the United Arab Emirates (UAE) and Sudan followed ally Saudi Arabia's lead in severing or downgrading ties with Iran as regional tensions escalate over the Saudi execution of a Shiite dissident cleric.

Bahrain and Sudan cut off diplomatic links with Iran on Monday, and the UAE downgraded its ties with the Islamic republic.

Riyadh meanwhile stepped up its own measures against Tehran, with Saudi Arabia's General Civil Aviation Authority announcing that airlines had been instructed to stop all flights to and from Iran.

The diplomatic fallout came only hours after the Saudi government broke off relations with Tehran, where protesters had stormed the Saudi embassy on Saturday night to protest Riyadh's execution of Shiite sheikh Nimr al-Nimr, a fiery critic of the Saudi authorities.

Al-Nimr was among 47 people executed by Saudi Arabia on Saturday after their convictions on terrorism-related charges.

There was international concern that the crisis could jeopardize diplomatic efforts to end the conflicts in Syria and Yemen, where Tehran and Riyadh back opposing sides.

UN Secretary General Ban Ki-moon spoke by phone with Saudi Foreign Minister Abel bin Ahmed al-Jubeir and his Iranian counterpart, Mohammad Javad Zarif, stressing that continued engagement was "in the interest of the region and beyond," his spokesman Stephane Dujarric said.

Ban told al-Jubeir that breaking off diplomatic ties with Iran was "deeply worrying."

Dujarric noted that one of the aims of the phone calls was to ensure the continued support of both countries for the peace process in Syria.

UN special envoy for Syria Staffan de Mistura was on his way to Riyadh and will also visit Iran later this week to "assess the implications of the recent developments," Dujarric said.

"He stresses the need to ensure that [the crisis in diplomatic relations] does not cause a chain of adverse consequences in the region," the spokesman said.

Abdallah bin Yahya al-Moallimi, Saudi Arabia's ambassador to the UN, said late Monday that his country was committed to the Syrian peace process and the diplomatic crisis with Iran would have "no effect" on those efforts.

"We will attend the next Syria talks and we are not going to boycott them because of Iran or anybody else," he said.

Al-Moallimi also defended the trials and subsequent executions saying they were internal matters under Saudi Arabia's sovereignty and said that criticisms stipulating that the trials had been unfair were "misinformed."

In Washington, White House spokesman Josh Earnest called "all sides to show restraint and not inflame tensions."

Bahrain and the UAE accused Iran of interfering in the internal affairs of Gulf and Arab countries, and Sudan said it was acting "in solidarity" with Riyadh.

Manama has ordered Iranian diplomats to leave its territory within 48 hours, the official Bahrain News Agency reported.

Bahrain charged Iran with "blatant and dangerous interference" in the internal affairs of Arab countries and support for terrorism.

The island state, whose Sunni ruling family is closely allied to Saudi Arabia, has seen two days of protests by members of its Shiite majority since al-Nimr's execution.

The UAE meanwhile pulled its ambassador from Tehran and said it was downgrading links to the level of charge d'affaire.

The country would also cut the number of Iranian diplomats on its soil, its Foreign Ministry said.

The oil- and gas-rich Arab Gulf states have long suspected Iran of seeking regional dominance and inciting unrest among Shiite populations in Bahrain and Saudi Arabia.

In recent months it has sent hundreds of troops to support the Saudi-backed government in Yemen against Iran-aligned Houthi rebels.

There have been reports of unrest in the eastern Saudi Arabian governorate of Qatif, al-Nimr's home region and a stronghold of the country's restive Shiite minority.

Al-Nimr was sentenced to death in 2014 on charges of causing sectarian strife and disobeying the ruler.

The trial was condemned as unfair by human rights groups and his supporters say he had only called for peaceful protests and criticized the ruling Al Saud family.

The execution has provoked a furious reaction from Shiite Muslims in the region. Iran's Supreme Leader Ayatollah Ali Khamenei threatened Saudi Arabia with "divine vengeance" for executing al-Nimr.

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