A top member of Iran’s parliament called Saudi Arabia’s King Salman a traitor to Islam on Thursday, a sharp escalation in rhetoric over Riyadh’s air strikes against an Iranian-allied militia that has seized much of neighboring Yemen.

Although a five-day humanitarian truce was largely holding, and aid was getting to some of the millions of Yemenis deprived of food, fuel and medicine, tensions were also mounting over an Iranian relief ship, which Saudi Arabia insisted on inspecting.

Yemen is the latest theater in Shi’ite Muslim Iran and Sunni Saudi Arabia’s tussle for power across the Middle East, which has played out along largely sectarian lines but has not yet led to direct confrontation between the Gulf powers.

Saudi Arabia and Arab states backed by the West have, since March 26, pounded Houthi rebels aligned with Tehran and forces loyal to ex-president Ali Abdullah Saleh, aiming to restore President Abd-Rabbu Mansour Hadi, now exiled in Riyadh.

While on a visit to Syria, Alaeddin Boroujerdi, head of the Iranian parliament’s national security and foreign policy committee, branded King Salman “traitor of the noble Haramayn”, or two sacred sites — a play on his official title of Custodian of the Two Holy Mosques.

The Saudi-led coalition has imposed an air and maritime blockade to stop weapons supplies reaching the Houthis and their allies, but Iran said it would not let the Saudis inspect an aid shipment it had sent toward Yemen.

Brigadier General Massoud Jazayeri, Deputy Chief of Iran’s Armed Forces, warned of war if the ship was attacked, and Hadi’s government withdrew its ambassador to Tehran.