From a Yemeni missile attack to the resignation of Lebanon's prime minister, the "Cold War" between Middle East rivals Saudi Arabia and Iran has been heating up.

Experts believe the risk of a direct military clash is low, but why have tensions escalated now and how will the crisis evolve?

AFP looks at five questions on the Riyadh-Tehran rivalry and its implications.

- What are the origins of the rivalry? -

Sunni powerhouse Saudi Arabia and Iran, the predominant Shiite power, have a long-standing rivalry based as much in geostrategic interests as religious differences.

Facing off across the Gulf, the two energy-rich powers have for decades stood on opposing sides of conflicts in the Middle East.

The Iranian revolution of 1979 and the advent of the Islamic Republic -- with its fiercely anti-American slant -- were perceived as a double threat to the conservative Sunni monarchies of the Arabian Peninsula, allied with the United States.

Saudi Arabia was a key financial backer of Iraqi dictator Saddam Hussein during his 1980-1988 war with Iran.

With Iraq weakened following the 1991 Gulf War, Saudi Arabia and Iran became "the two main regional powers", said Clement Therme, a researcher at the International Institute for Strategic Studies (IISS).

- How have tensions escalated recently? -

The latest round of tensions began when Riyadh and Tehran broke off diplomatic relations in January 2016, after Iranians stormed Saudi Arabia's embassy and consulate in response to the execution of a prominent Shiite cleric.

That followed the 2015 nuclear deal between Tehran and six world powers, which Riyadh feared was a step towards ending Iran's international isolation.

Rhetoric between the two grew increasingly belligerent, including over Saudi Arabia's Gulf neighbour Qatar. Riyadh and several of its Sunni allies broke off diplomatic relations with Qatar in June 2017, accusing Doha of support for extremism and links with Iran, claims that it denies.

On the first weekend of November, the animosity reached new heights. First, the Saudi-supported prime minister of Lebanon, Saad Hariri, in a broadcast from Riyadh announced his resignation, blaming Iran's "grip" on his country via Shiite movement Hezbollah.

Several hours later, Saudi Arabia said its air defences near Riyadh intercepted and destroyed a missile fired from Yemen, where a Saudi-led coalition is battling Iran-backed Shiite rebels.

That set off a fierce war of words between Riyadh and Tehran, with Saudi Arabia's powerful Crown Prince Mohammed bin Salman accusing Iran of "direct military aggression". Tehran denied any involvement in the missile attack, with President Hassan Rouhani warning that Iranian "might" would fend off any challenge.

- Why now? -

"The main cause of the current tensions is related to the proxy confrontation between Iran and Saudi Arabia," Therme said, pointing to wars in Iraq, Syria and Yemen.

Recent months have seen changes in these confrontations that appear to have brought the tensions to a head.

In Iraq and Syria, the increasingly successful campaign against the Islamic State group has changed the situation on the ground. Offensives in both countries have forced the jihadists from nearly all the territory they seized in mid-2014.

As the threat from a common enemy "has imploded, tensions between these historic adversaries have escalated", said Max Abrahms, professor of political science at Northeastern University in Boston.

As Iraq looks to a post-IS era, Riyadh has been taking steps to build stronger ties with the country's Shiite-dominated government. A flurry of visits between the two countries this year saw talk of a warming of ties, including a trip by Prime Minister Haider al-Abadi to Riyadh in late October.

In Syria, meanwhile, the Iran-backed government of President Bashar al-Assad has over the past year managed to reassert control over large parts of the country by defeating, among others, rebel groups backed by Riyadh.

"The Saudi-Iranian rivalry has become the organising principle for Mideast alliances, reminiscent of how the Cold War divided countries along US and Soviet lines," Abrahms said.

Analysts said the election as US president a year ago of Donald Trump has also contributed to the rise in tensions. Trump's open hostility towards Tehran has "released anti-Iranian energies in the Arabian Peninsula" and emboldened Riyadh, Therme said.

source: AFP