Russian President Vladimir Putin will visit Tehran Monday for talks sure to sharpen attention on his alliance with Iran in Syria after the Islamic State group's deadly attacks in Paris. The trip coincides with a major summit in Iran's capital of gas exporting countries, but Putin's meeting with Iran's supreme leader Ayatollah Ali Khamenei is likely to dominate.
An escalation in fighting in Syria, where Russia has propped up President Bashar al-Assad against IS and Western-backed rebels, most recently with air strikes, has pulled it closer to Iran which is co-ordinating pro-Assad fighters on the ground. Russia is also 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, but 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, years after Iran paid for the contract.
Moscow said the recent nuclear deal between Iran and the P5+1 group - Britain, China, France, Russia and the United States plus Germany - would allow the deal to be completed.
But Iran and Putin's support of Assad has come under greater scrutiny since IS claimed responsibility for killing 130 civilians in the gun and bomb attacks in Paris, shortly after the jihadists said they blew up a Russian airliner in Egypt, killing 224. The United Nations passed a motion Friday calling for action against IS after the Paris atrocities which have sparked fears of similar attacks elsewhere in Europe. Monday's trip is Putin's first to Iran since 2007. Talks will focus on "issues in bilateral relations, including atomic energy, oil and gas and military-technical cooperation", his top foreign policy aide Yury Ushakov said Friday.
The fighting in Syria, where more than 250,000 people have died since 2011, shows no signs of ending.
The UN Security Council on Friday authorised countries to "take all necessary measures" to fight IS in a France-sponsored resolution that won backing from the five permanent members, including Russia.
Moscow's aim of an international coalition made up of Iran, Jordan and other regional and Western countries against IS 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, has backed Russia's intervention, and says only the Syrian people, not outside powers, can choose to dump Assad in elections following a cease-fire.
Although Iran and Russia have not always seen eye to eye, both are now interested in limiting US influence in the Middle East.
Moscow recently announced opening a $5 billion credit line for Iran and help for Iran's struggling banking sector is also expected.
Russian companies are eyeing business opportunities after sanctions on Iran are lifted, expected in the next two months as the nuclear deal reaches its "implementation" stage.
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.