Iran's Supreme Leader Ayatollah Ali Khamenei called the leadership in Saudi Arabia "worthless, inept and villainous" as he heaped scorn on the ultra-conservative kingdom. Khamenei said Riyadh is bound to witness a "certain downfall" in the coming years as they continue to oppress their own people.

The Iranian supreme authority, who holds the highest office in Iran and has a final say on all the political affairs, was addressing an Islamic gathering on Saturday (27 May) to mark the first day of Ramadan, the fasting month which is considered holy by Muslims. Iran, a Shia-dominated regional heavyweight, has a fractious relationship with the Sunni powerhouse of Saudi Arabia. Their relations are strained over a series of religio-political issues such as the way Islam is interpreted, the Islamic world's leadership and oil exports.

While speaking to the so-called Quranic group, Khamenei chided the Saudi leadership for its close ties with the US and the recently approved arms deal. Describing the Saudi monarch as "idiots", Khamenei said Riyadh acts like the "milk cows for the Americans".

"Today the Muslim world, like all other societies, is in grave danger. Among the Muslim world, a group of worthless, inept and villainous people are ruling over a community of the Muslim nation, namely the Saudi government; and, reason lies in refusal to follow Quran and lack of belief in the truth," Khamenei said.

He cited the ongoing conflict in Yemen and the tense situation in Bahrain to condemn the Saudi rulers' role in contributing to regional instability. Both Iran and Saudi Arabia are engaged in a bitter proxy conflict in Yemen, where the Shia-aligned Houthi rebels are backed by Tehran. The rebels are being pounded by Saudi forces in the fight to capture control of Yemen.

Only in early May, Iran's defence minister threatened to completely annihilate Saudi Arabia except for Mecca and Medina, the two sites which are regarded as holy by Muslims.