TEHERAN (AFP) - Russia and Iran jointly oppose "external attempts" to bring regime change in Syria, a Kremlin official said Monday in Teheran after President Vladimir Putin met supreme leader Ayatollah Ali Khamenei.

The comments were a direct rebuff of repeated demands from the United States, France, Britain and Saudi Arabia that President Bashar al-Assad step down and play no future role in war-torn Syria.

On his first trip to Iran in eight years, Putin, accompanied by his Foreign Minister Sergei Lavrov, went straight into a meeting with Khamenei, the Islamic republic's ultimate authority.

Describing the 90-minute encounter as "quite constructive" and longer than planned, a Kremlin spokesman said the two countries had "unity of views" on Syria.

Russia and Iran are against "external attempts to dictate scenarios of political settlement" in the conflict-wracked state, and only Syria's people could decide to dump Assad in elections following a ceasefire.

Khamenei said the US had a "long-term plan" to dominate Syria and the Middle East which would "disadvantage all countries, especially Iran and Russia".

"This threat should be neutralised wisely and with closer interaction," he was quoted as saying in a statement.

Putin and Khamenei met before the Russian leader headed to a major summit of gas exporting countries in the Iranian capital.

Iran and Russia have become increasingly allied in Syria providing support that has propped up Assad's government and forces since an uprising erupted in 2011.

What began as a conflict between Assad's army and Western- and Gulf-backed rebels has since spiralled into a multi-faceted war that has killed more than 250,000.

Attention is currently focused on stopping ISIS (Islamic State in Iraq and Syria), who last year seized large parts of Syria before surging into Iraq.

The threat from ISIS has taken on new potency and spread into Europe since the extremists committed coordinated gun and bomb attacks in Paris on November 13, killing 130 people.

For Russia, defending Assad and confronting ISIS has become more important since the militants blew up a Russian airliner over Egypt's Sinai Peninsula on Oct 31, killing all 224 on board.

Russia had one month earlier launched a wave of air strikes in support of Assad, whose Alawite sect, an offshoot of Shiite Islam, has close ties to Iran, the region's main Shiite power.

Both Iran and Russia, which has a major sea port base in Syria, are seeking to limit US leverage in the Middle East.

This shared goal has seen Iran send commanders from its elite Revolutionary Guards to support and advise Assad's forces, with Teheran coordinating a collection of Shiite militias on the ground.

On the economy, Russia and Iran are also looking at bigger deals once sanctions are lifted under the July 14 nuclear deal between Teheran and world powers, including Moscow.

Putin stood alongside Iran's President Hassan Rouhani at the opening of the gas summit, attended also by counterparts Nicolas Maduro of Venezuela, Muhammadu Buhari of Nigeria and Evo Morales of Bolivia.

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. Moscow, however, later provided Saddam Hussein with weapons during the Iran-Iraq war.

A long-delayed delivery of an advanced missile defence system, the S-300, is due from Russia by the end of 2015.

During Putin's visit the Kremlin announced it had lifted a ban on Russian firms working on uranium enrichment at Iran's atomic sites, following the nuclear deal.

Putin's trip 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.

But Moscow's aim of an international coalition made up of Iran, Jordan and other regional and Western countries against ISIS is coming up against deadlock over Assad's future, which recent peace talks in Vienna failed to break.

The US, France, Britain and Sunni Arab countries plus Turkey all want Assad to go, and have said the Russian bombing was aimed at destroying "moderate rebels".

Russian companies are eyeing opportunities in Iran after sanctions are lifted, a step expected in the next two months as the nuclear deal reaches its "implementation" stage.