Iranian Foreign Minister Javad Zarif railed at Saudi Arabia’s “barbarism” and support for “extremism” in a New York Times op-ed published Sunday, the latest blow in an escalating feud between the two Gulf states.

The senior Iranian diplomat, who led the Islamic Republic’s delegation in the nuclear talks with world powers, accused Riyadh of “standing in the way of constructive engagement” and brainwashing those who carried out terrorist attacks against the United States.

“Let us not forget that the perpetrators of many acts of terror, from the horrors of Sept. 11 to the shooting in San Bernardino and other episodes of extremist carnage in between, as well as nearly all members of extremist groups like Al-Qaeda and the Nusra Front, have been either Saudi nationals or brainwashed by petrodollar-financed demagogues who have promoted anti-Islamic messages of hatred and sectarianism for decades,” Zarif wrote.

The opinion piece ran amid heightened tensions between the two Middle Eastern powers in the wake of Saudi Arabia’s execution of prominent Shiite cleric and activist Nimr al-Nimr. Saudi Arabia’s embassy in Tehran and its mission in Mashhad, Iran’s second largest city, were attacked by mobs and set on fire after the killing. Diplomatic ties were cut by the Saudis 24 hours later.

Zarif wrote that President Hassan Rouhani “has repeatedly declared that Iran’s top foreign policy priority is friendship with our neighbors, peace and stability in the region and global cooperation, especially in the fight against extremism.”

Riyadh officials, on the other hand, “not only continue to impede normalization but are determined to drag the entire region into confrontation,” he charged, accusing “the Saudi government or its surrogates” of attacking Iranian diplomats in Lebanon, Pakistan and Yemen, harassing its nationals traveling to Mecca, and inciting violence against Shiite Muslims.

Zarif denounced Riyadh’s “barbarism” in its execution of prisoners and, in a thinly veiled reference, accused Saudi Arabia of supporting the ultra-radical Islamic State group.

The Islamic Republic, for its part, executed nearly 1,000 prisoners in 2015, according to the Iran Human Rights Documentation Center, what a UN investigator called “alarming” in October. Ahmed Shaheed said in a report to the UN General Assembly that most of the executions violated international laws banning the use of capital punishment for non-violent offenses. Tehran also supports Syrian President Bashar Assad, and bankrolls the Lebanese Shiite terrorist group Hezbollah.

“Iran has no desire to escalate tension in the region. We need unity to confront the threats posed by extremists,” Zarif said in the op-ed.

“The Saudi leadership must now make a choice: They can continue supporting extremists and promoting sectarian hatred; or they can opt to play a constructive role in promoting regional stability. We hope that reason will prevail,” he wrote.

Earlier on Sunday Zarif accused Saudi Arabia of using its row with Tehran to “negatively affect” peace talks on the Syrian conflict.

“Saudi Arabia’s approach is to create tension intended to negatively affect the Syrian crisis,” he said in a Foreign Ministry statement. “We will not allow Saudi actions to have a negative impact.”

The statement coincided with a visit to Tehran for talks by Staffan de Mistura, the UN peace envoy on Syria, one week after Saudi Arabia cut diplomatic ties with Iran.

The UN Security Council is backing an 18-month plan to end Syria’s nearly five-year war and the roadmap was the result of recently launched international talks aimed at ending the conflict.