Relations between Middle East heavyweights Saudi Arabia and Iran have been fraught for decades as they have sparred over religion, oil and regional supremacy.

Each rival considers itself to be the guardian of one of the two main branches of Islam -- Saudi Arabia is a Sunni kingdom and Iran is Shiite.

With tensions escalating this week after Riyadh accused Tehran of "direct military aggression" linked to the war in Yemen, here is a look at the high points of tension.

- Oil -

In 1973, during the Arab-Israeli war, the Organization of Petroleum Exporting Countries (OPEC) imposes a steep rise in oil prices.

Arab producers decree an embargo on countries that are considered pro-Israeli, including the United States.

Tensions spike: Iran wants an even higher crude price to finance its ambitious industrial development plans; Riyadh wants to prevent a too-strong increase to spare its US ally.

- Iranian revolution -

The Islamic Republic of Iran is established in April 1979 after a revolution which overthrew the pro-Western monarchy. The region's Sunni nations accuse Iran of seeking to "export" the revolution to them.

- Iran-Iraq war -

In September 1980, Iraq attacks Iran, triggering an eight-year war. Saudi Arabia financially backs the Iraq regime against Iran and encourages other Sunni Gulf countries to do likewise.

- Deadly crackdown at Mecca -

Saudi security forces in Mecca crack down in July 1987 on an unauthorised anti-US protest by Iranian pilgrims. More than 400 people, mostly Iranians, are killed.

Angry Iranians loot the Saudi embassy in Tehran, and in April 1988, Saudi Arabia breaks off diplomatic relations with Iran. Its pilgrims are absent until 1991.

- Smoothing over differences -

The election of reformist Iranian president Mohammad Khatami in May 1997 leads to improved relations with Riyadh, to which he makes a historic visit in May 1999.

But the US-led invasion of Iraq in 2003 sparks fresh tension by removing the ruling Baath party and allowing the majority Shiites to take over, bringing the country under Iran's sphere of influence.

- Arab Spring -

As Arab Spring demonstrations sweep the Middle East in 2011, Riyadh sends soldiers to neighbouring Bahrain where protests by the Shiite majority have broken out.

Riyadh accuses Tehran of stoking tension in the Sunni-ruled island state.

The rival capitals square off again in 2012 as the Syria crisis erupts.

Iran backs President Bashar al-Assad and provides him with military forces and funds to battle Sunni rebels.

Saudi Arabia backs the rebels, but also joins a US-led coalition fighting the Sunni extremist Islamic State group.

Saudi Arabia and Iran also take opposing sides in the Yemen conflict: in March 2015, Riyadh forms a Sunni Arab coalition to intervene in support of the Yemeni president, while Tehran backs the Shiite Huthi rebels.

- Hajj stampede -

A stampede at the hajj annual pilgrimage in September 2015 leaves around 2,300 foreign pilgrims dead, including hundreds of Iranians.

Iran's supreme leader Ayatollah Ali Khamenei says afterwards that the Saudi ruling family does not deserve to manage Islam's holiest sites.

- Cleric executed -

In January 2016, Saudi Arabia executes prominent Shiite cleric Nimr al-Nimr, a driving force behind anti-government protests, for "terrorism".

Iran is furious. Saudi diplomatic missions in Iran are attacked by protesters and Riyadh breaks off relations with Tehran.

- Hezbollah -

Lebanon's powerful Shiite militia Hezbollah, an Iran ally, is in March 2016 classified as "terrorist" by the Gulf Arab monarchies.

This is after its chief accused Saudi Arabia of seeking to stoke a "revolt" between Sunni Muslims and Shiites.

In November 2017, it is from Riyadh that Lebanese Prime Minister Saad Hariri announces his resignation, citing Iran's "grip" on the country through Hezbollah.

- Qatar crisis -

In June 2017, Saudi Arabia and its allies break off diplomatic relations with Qatar, accusing it of fostering too close ties with Iran and backing extremism.

- Nuclear accord -

In October 2017, Saudi Arabia says it backs US President Donald Trump's "firm strategy" on Iran after he refuses to certify the 2015 Iranian nuclear deal.

source: AFP