The diplomatic row that has seen Saudi Arabia and the kingdom’s Sunni allies cut ties with Iran widened Tuesday as Kuwait recalled its ambassador to Tehran in the face of growing international concern.

Joining Riyadh and others in taking diplomatic action, Kuwait said it was downgrading ties with Iran over a weekend attack on the Saudi embassy in Tehran.

Kuwait's move came after the UN Security Council strongly condemned the attack, carried out by protesters angry over Saudi Arabia's execution of a prominent Shia cleric.

Tensions between Saudi Arabia, the main Sunni power, and Shia-dominated Iran have erupted this week into a full-blown diplomatic crisis, sparking widespread worries of regional instability.

Washington and other Western powers have called for calm amid fears the dispute could raise sectarian tensions across the Middle East and derail efforts to resolve conflicts from Syria to Yemen.

The Security Council joined those calls late on Monday, issuing a statement urging all sides to “take steps to reduce tensions in the region.”

The statement by the 15-member council condemned “in the strongest terms” the attacks, which saw protesters firebomb the Saudi embassy in Tehran and its consulate in Iran's second-biggest city Masshad.

But the council made no mention of the event that set off the crisis — Saudi Arabia's execution on Saturday of Sheikh Nimr al-Nimr, a cleric and activist whose death sparked widespread protests among Shias in Iran, Iraq and across the Middle East.

Saudi Arabia cut off diplomatic ties with Tehran in protest at the attacks on Sunday and has severed air links with Iran.

Some of its allies among Sunni Arab states followed suit, with Bahrain and Sudan breaking off ties and the United Arab Emirates downgrading relations on Monday.

Kuwait said Tuesday the embassy attacks “represent a flagrant breach of international agreements and norms and a grave violation of Iran's international commitments.”

Iranian President Hassan Rouhani has condemned the attacks and Tehran's mission to the UN vowed in a letter to the Security Council to “take necessary measures to prevent the occurrence of similar incidents in the future.”

But on Tuesday he doubled down on criticism of Saudi Arabia’s execution on Nimr, stating that the kingdom “cannot cover its crime by severing ties with Iran.”

US Secretary of State John Kerry called his Iranian and Saudi counterparts on Monday to urge calm as European leaders raised concerns and Moscow offered to act as an intermediary.

UN Secretary-General Ban Ki-moon also spoke by phone with the Saudi and Iranian foreign ministers to urge them to “avoid any actions that could further exacerbate the situation,” Ban's spokesman Stephane Dujarric said.

“A breakdown of relations between Riyadh and Tehran could have very serious consequences for the region,” Dujarric said.

The UN envoy for Syria, Staffan de Mistura, headed to Riyadh and Tehran to defuse tensions, worried that the row would undermine growing efforts to resolve that country's conflict.

Iran and Saudi Arabia are on opposing ends of a range of crucial Middle East issues, including the war in Syria — where Tehran backs President Bashar al-Assad's regime and Riyadh supports rebel forces — and Yemen where a Saudi-led coalition is battling Shiite insurgents.

Despite the fears, Saudi Arabia's ambassador to the UN, Abdallah al-Mouallimi, insisted the row would not have an impact on efforts to resolve regional conflicts.

“From our side, it should have no effect because we will continue to work very hard to support the peace efforts in Syria and Yemen,” Mouallimi told reporters.

He said Riyadh would attend upcoming talks on Syria, but took a swipe at Iran's role in the nearly five-year war there, saying: “They have been taking provocative and negative positions …and I don't think the break in relations is going to dissuade them from such behavior.”

Still, the crisis “will further diminish already low expectations,” in particular for the talks on Syria expected to take place this month, according to Noah Bonsey, a senior analyst at the International Crisis Group.

“Ultimately, reaching a political resolution in Syria would require key states backing each side to make reciprocal concessions, and pressure their Syrian allies to do the same,” he said. “For now, things are moving in the opposite direction.”

The spike in tensions comes after Iran last year secured a historic nuclear deal with world powers led by the United States, sparking major concern in longtime US ally Riyadh.

The diplomatic row saw shares on Gulf exchanges drop and oil prices rebound amid fears instability could affect supplies from Saudi Arabia and Iran, both members of the Organization of the Petroleum Exporting Countries (OPEC).

Nimr, one of 47 men executed on Saturday, was a driving force behind 2011 anti-government protests in eastern Saudi Arabia.

He was arrested in 2012 after calling for two Saudi governorates to be separated from the kingdom. Riyadh's interior ministry at the time described him as an “instigator of sedition.”

Al Jazeera and Agence France-Presse