Russian President Vladimir Putin has met Iranian Supreme Leader Ayatollah Ali Khamenei in Tehran, who called for closer ties between the two countries.
During their meeting on Monday, Khamenei said that US policies in the Middle East are a threat to both Iran and Russia.
The ongoing civil war in Syria has evolved into a wider proxy struggle between global powers, with Russia and Iran supporting Syrian President Bashar al-Assad while Western powers, Turkey and Gulf Arab states support Syrian rebel groups.
"The Americans have a long-term plot and are trying to dominate Syria and then the whole region ... This is a threat to all countries, especially Russia and Iran," Khamenei said, according to his website, at the meeting on the sidelines of the Gas Exporting Countries Forum (GECF) Summit in Tehran.
"The United States is now trying to achieve its failed military objectives in Syria by political means," he added, referring to proposed peace talks to end the civil war in Syria.
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Al Jazeera's Rory Challands, in Moscow, said that while both countries still oppose Western intervention in Syria, it is unclear whether disagreements have arose on other issues in that country.
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Our correspondent said that Western governments are looking for "any sign of disagreements about what the fate is for Assad and whether they are still committed that he is the man for the future".
At a meeting in Vienna this month following deadly attacks in Paris and Beirut, world powers, including Russia, the United States and countries from Europe and the Middle East, agreed on a political process in Syria leading to elections within two years, but differences remained on key issues such as Assad's future.
A Kremlin spokesman was quoted by Interfax news agency as saying that Putin and Khamenei had agreed at their talks that global powers should not impose their political will on Syria.
Putin, on his first visit to Iran since 2007, presented an old edition of the Quran to Khamenei, the Iranian leader's website said, publishing photos of the book.
Khamenei praised Putin for "neutralising Washington's plots" and said economic relations between the two countries could "expand beyond the current level".
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Tehran and Moscow have stepped up ties following a landmark nuclear deal in July between Iran and six world powers including Russia and the United States. Under the deal, Tehran agreed long-term curbs on its nuclear programme in exchange for an easing of economic sanctions.
On Monday, Putin also relaxed an export ban on nuclear equipment and technology to Iran.
Iran's ambassador to Russia said that Moscow had started the process of supplying Tehran with an S-300 anti-missile rocket system.
Russia and Iran are undertaking joint military action in support of Assad. Backed by Russian air strikes, hundreds of Iranian troops have arrived since late September to take part in a major ground offensive planned in western and northwestern Syria, their biggest deployment in the country to date.
Source: Al Jazeera and agencies