Russian President Vladimir Putin arrived Monday in Tehran for talks with Iran about the devastating conflict in Syria where the two nations are allied in support of the Damascus regime.
On his first trip to Iran in eight years, Putin met supreme leader Ayatollah Ali Khamenei, the country's ultimate authority, who has backed Syrian President Bashar al-Assad since an uprising broke out in 2011.
What began as a conflict between Assad's forces and Western- and Gulf-backed rebels has since spiralled into a civil war and multi-faceted fighting that has killed more than 250,000 people.
Attention is currently focused on stopping ISIS group, who last year took control of large parts of Syria and surged into Iraq, from breaching Assad's defences and taking Damascus.
The threat from ISIS has taken on new potency and spread into Europe since the jihadists committed coordinated gun and bomb attacks in Paris 10 days ago, killing 130 people.
Russia had one month earlier launched a wave of airstrikes in support of Assad, whose Alawite sect, an offshoot of Shia Islam, has close ties to Iran, the Middle East's main Shia power.
Both Iran and Russia, which has a major sea port base in Syria, are seeking to limit US leverage in the Middle East.
Their shared goals have seen Iran send commanders from its elite Revolutionary Guards to support and advise Assad's forces, with Tehran coordinating a collection of Shia militias on the ground.
Putin's trip coincides with a major summit in Tehran of gas exporting countries but his talks with Khamenei are likely to dominate.
Russia is emerging as a long-term arms partner for Iran, despite the countries having a complicated history over territory, oil, business and communism.
The former Soviet Union was the first state to recognise Iran as an Islamic republic after the 1979 revolution, though Moscow later provided Saddam Hussein with weapons during the Iran-Iraq war in the 1980s.
A long-delayed delivery of an advanced missile defence system, the S-300, is due from Russia by the end of 2015.
Putin's visit was planned before the UN Security Council on Friday authorised countries to "take all necessary measures" to fight ISIS in a France-sponsored resolution one week after the Paris attacks.
Moscow's aim of an international coalition made up of Iran, Jordan and other regional and Western countries against ISIS is coming up against a deadlock over Assad's future, which recent peace talks in Vienna failed to break.
The United States and Sunni Arab countries, most vocally Saudi Arabia, plus Turkey, all want Assad to go, and have said the Russian air strikes were aimed at destroying "moderate rebels" fighting the Syrian president since 2011.
Iran, however, says only the Syrian people, not outside powers, can choose to dump Assad in elections following a ceasefire.
Russian companies are eyeing business opportunities in Iran after sanctions are lifted, a step expected in the next two months as Tehran's July 14 nuclear deal with world powers reaches its "implementation" stage.
Moscow has announced opening a $5 billion credit line for Iran and help for Tehran's struggling banking sector is also expected.
Several leaders from a dozen gas producing countries -- who together hold 67 percent of proven reserves -- will be at Monday's summit.
Iran's President Hassan Rouhani, with whom Putin will also hold talks, is hosting seven presidents, including Nicolas Maduro of Venezuela, Muhammadu Buhari of Nigeria and Evo Morales of Bolivia.