Kuwait has recalled its envoy from Iran, a day after Bahrain and Sudan—two other allies of Saudi Arabia—severed diplomatic relations with Tehran, and a third, the United Arab Emirates, downgraded them.

The steps, each following the kingdom’s own cutting off diplomatic relations with the Islamic republic, follow the storming of the Saudi Embassy in Tehran and an attack on its consulate in Mashhad in an apparent response to the execution by Saudi Arabia of a prominent Shia cleric over the weekend.

Iran fired back on Tuesday, saying Saudi Arabia can’t “cover its crime” by severing diplomatic ties, further raising tensions in the region, and prompting fears of a wider sectarian conflict between Sunnis, who represent the overwhelming majority of the Muslim world, and Shiites, who are a majority in Iran and a significant minority in other places. The U.S. and its allies, as well as Russia, Turkey, and China, meanwhile, appealed for calm.

A statement from the Kuwaiti Foreign Ministry said its ambassador to Tehran was being recalled because of the “torching and sabotage” of the Saudi facilities in Iran.

“Such action constitutes a flagrant breach of international conventions and violation of Iran’s international commitment over security and safety of diplomatic missions on its lands,” the Foreign Ministry said.

Iran, meanwhile, sought to draw a distinction between Saudi Arabia’s execution of Sheikh Nimr al-Nimr, the Shia cleric, and the protests that followed.

“Of course, the Saudi government, in order to cover up its crime of beheading a religious leader has resorted to a strange measure and has severed its ties with the Islamic Republic,” President Hassan Rouhani said Tuesday.

But Kazem Jalali, a prominent Iranian lawmaker, criticized the actions of the protesters as, in the words of the state-run Islamic Republic News Agency, “wrong and impulsive.” Iran even arrested some of the protesters who stormed the Saudi facilities.

In Washington on Monday, John Kirby, the State Department spokesman, urged calm.

“We continue to believe that diplomatic engagement and direct conversations are essential to work through differences,” he said.

The U.S. and others criticized Nimr’s execution, but also condemned the nature of the protests in Iran. The UN condemned the protests, but made no mention of the execution.

Nimr, a Shia cleric, was a critic of the Saudi monarchy and had led protests in the eastern part of the country, where many Saudi Shiites live. His execution sparked protests by Shiites across the world, including in Iraq, Bahrain, Pakistan, and Iran. And, as we pointed out on Monday:

Although relations between Saudi Arabia, which is mostly Sunni, and Iran, which is predominantly Shiite, have never been warm, the tensions mark the worst deterioration in ties in recent years. It’s been some time coming: The two countries are on opposite sides of the civil war in Yemen, where the Saudis support the government and the Iranians the Houthi rebels; and in Syria, where Riyadh supports some rebel groups and Iran the government of President Bashar al-Assad. They have also parried in Bahrain, where a Sunni ruler governs a mostly Shiite population, and in Iraq, where Iran holds considerable influence over the predominantly Shiite government; and traded barbs over the death toll of the stampede at the Hajj last September.

The sometimes-violent tensions between Sunnis and Shiites are almost as old as Islam itself. Indeed, the latest tensions have prompted fears of a region-wide sectarian conflict, but they also complicate U.S. efforts to forge a global coalition against the Islamic State group in the region.

ISIS, as the Islamic State is also known, controls territory across Iraq and Syria, and Western nations see the group’s defeat as a pivotal step toward bringing about a semblance of stability to the Middle East. Worsening relations between the region’s two most powerful Muslim nations are likely to make that more difficult, though both countries ostensibly oppose ISIS.