Flames rise from Saudi Arabia’s embassy during a demonstration at in Tehran.

Attacks on diplomatic missions can never be a legal means of protest, Russia said on Monday, after Iranian demonstrators stormed the Saudi embassy in Tehran in protest at Riyadh’s execution of a senior Shiite cleric.

Russia’s foreign ministry expressed “serious concern” about the worsening situation in the region and called on the two countries and others “to show restraint and avoid any steps that would complicate the situation and lead to a rise in tensions.”

Earlier, Russia is ready to serve as an intermediary to resolve the dispute between Saudi Arabia and Iran that saw the kingdom break off diplomatic relations with Tehran, a Russian foreign ministry source told AFP on Monday.

“Russia is ready to serve as an intermediary between Riyadh and Tehran,” the source said, without providing any specifics about Moscow’s potential role in resolving the crisis.

Another unnamed Russian diplomatic source quoted by TASS news agency said Moscow was ready to host the Saudi and Iranian foreign ministers — Adel al-Jubeir and Mohammad Javad Zarif — for talks.

“If our partners Saudi Arabia and Iran show they are ready and willing (to meet), our initiative will remain on the table,” the source said.

Saudi Arabia announced Sunday it was severing its ties to Iran after its embassy in Tehran was firebombed in protest at the kingdom’s execution of Shiite cleric Sheikh Nimr al-Nimr.

Nimr was a force behind 2011 anti-government protests in eastern Saudi Arabia.

The oil-rich rivals have also been divided over the nearly five-year war in Syria, where Iran is backing the regime, and the conflict in Yemen where a Saudi-led coalition is battling Shiite rebels.

Russian Foreign Minister Sergei Lavrov hosted both Jubeir and Zarif individually last year for talks on the Syrian crisis as Moscow pushed for the creation of a broad coalition to fight Islamic State jihadists in Syria.

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