UNITED NATIONS — UN Secretary-general Ban Ki-moon spoke with the foreign ministers of Iran and Saudi Arabia Sunday to try to quell new tensions between the two Middle Eastern powerhouses.

The effort comes in the wake of Saudi Arabia’s execution Saturday of a prominent Shia cleric and the response by some Iranians to toss Molotov cocktails at Saudi Arabia’s embassy in Tehran.

Because the skirmish between Sunni Saudi Arabia and Shia Iran threatened the goal of achieving an end to the civil war in Syria, Ban’s special envoy, Staffan de Mistura, rushed to the capitals, Riyadh and Tehran, to try to save the delicate political process that may be the best hope yet to stop the five-year-old war that has claimed nearly 300,000 lives.

While those trips by de Mistura had been planned in advance, they took on a greater sense of urgency, said Stephane Dujarric, a spokesman for Ban, at a news conference at UN headquarters in Manhattan Monday. Dujarric said Ban called for leaders in the region “to avoid adding fuel to the fire.”

White House spokesman Josh Earnest said the United States was urging Saudi Arabia and Iran to not let the rupture sabotage the progress made through talks in Vienna over Syria’s future.

“We’re urging all sides to show some restraint and not further inflame tensions that are on vivid display in the region,” he said. “It is so clearly in the interests of both countries to advance a political solution to the situation inside of Syria.”

Saudi Arabia severed diplomatic ties with Iran over the execution, and several countries, including Bahrain, Sudan and United Arab Emirates, have also downgraded relations with Iran.

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Sheik is Nimr al-Nimr and 46 other prisoners were executed after trials that Ban said “raised serious concerns over the nature of the charges and the fairness of the process,” prompting retaliation against Saudi Arabia’s diplomatic premises in Tehran.

Ban spoke to Adel bin Ahmed al-Jubeir, foreign minister of Saudi Arabia and Mohammad Javad Zarif, foreign minister of Iran, stressing his disappointment over the executions, as Ban has long opposed the death penalty, as well as the attack hours later on the embassy, calling it “deplorable.”

The Associated Press reported that Iran expressed “regret” over the attack and that it will spare no effort in arresting and prosecuting those responsible and “will take necessary measures to prevent the occurrence of similar incidents in the future,” read the letter from Iran’s U.N. envoy, Gholamali Khoshroo.