The heated row between Saudi Arabia and Iran is more than just a spat between regional rivals.

It is an ancient conflict between Sunni and Shia Muslim powers that threatens regional stability and to derail peace initiatives in Syria and Yemen that showed faint signs of a breakthrough toward the end of 2015.

It could also upset the fight against Islamist extremists calling themselves Islamic State, ISIS or Daesh as they are also known who want to set up a Sunni Muslim caliphate.

It is based on brute force and principles no longer considered human but its extremist ideas are said to have roots Saudi Arabia.

Saudi Arabia on Jan 3 officially cut ties with Iran over the storming of the Saudi Embassy in Tehran, following the execution of Saudi Shiite cleric Nimr Al-Nimr.

Foreign Minister Adel Al-Jubeir told a news conference Iran's diplomatic mission and related entities in Saudi Arabia had been given 48 hours to leave, Arab News reported.

He said Riyadh would not allow Tehran to undermine the Kingdom's security.

Al-Jubeir called Iran a regional menace for its smuggling of arms and explosives and its previous harboring of Al-Qaeda militants.

In Tehran, angry crowds had hurled Molotov cocktails and stormed the Saudi Embassy. Fires were seen burning inside the building.

Al-Jubeir said the aggressive statements of the Iranian regime encouraged the attacks on Saudi missions.

He accused Iran of having a history of supporting terrorism, citing its support to the bloody regime of Bashar Assad in Syria. Al-Jubeir said the Saudi Arabia rejects all criticism of its justice system.

The unrest erupted after Saudi authorities announced that Sheik Nimr Baqr al-Nimr, 56, was among 47 people put to death.

Some of those executed were killed by firing squad, others by beheading, according to a statement from Saudi Arabia's Interior Ministry.

Most were Sunnis accused of participating in al-Qaida attacks in the kingdom, The Washingon Post reported.

Nimr, however, was one of four Shiites put to death for political activism and a leader of anti-government demonstrations in the mainly Shiite east of the country in 2011.

The Iranian Foreign Ministry warned that there would be repercussions.

"The Saudi government will pay a heavy price for adopting such policies," said Foreign Ministry spokesman Hossein Jaber Ansari.

'IRRESPONSIBLE EXECUTION'

He called the execution "the depth of imprudence and irresponsibility" on the part of the Saudi government, according to Iranian news agencies.

Iran summoned the Saudi charge d'affaires in Tehran to complain about the execution, and Saudi Arabia did the same by calling in the Iranian ambassador in Riyadh to protest the "hostile" remarks made in Iran by officials.

The execution also reignited unrest in both Saudi Arabia and neighboring Bahrain, after years of calm following clampdowns on demonstrations in 2011.

The U.S. State Department, in a statement, called on Saudi Arabia "to respect and protect human rights" and to permit "peaceful expression of dissent."

"We are particularly concerned that the execution of prominent Shia cleric and political activist Nimr al-Nimr risks exacerbating sectarian tensions at a time when they urgently need to be reduced," the State Department said.

"In this context, we reiterate the need for leaders throughout the region to redouble efforts aimed at deescalating regional tensions."

Shiites around the world expressed outrage, potentially complicating a surge of US diplomacy aimed at bringing peace to the troubled region, The Washington Post quoted Toby Matthiesen, an expert on Saudi Arabia at the University of Oxford as saying.

"Nimr had become a household name amongst Shiite Muslims around the world. Many had thought his execution would be a red line and would further inflame sectarian tensions," he said. "So this will complicate a whole range of issues, from the Syrian crisis to Yemen."