Russia denied Thursday having a "hidden agenda" on Syria as it launched a fresh round of crisis diplomacy by hosting the top diplomats of Iran and Damascus ahead next week's historic peace talks.

Iranian Foreign Minister Mohammad Javad Zarif arrived in the Russian capital from Damascus on the same jet as Syrian Foreign Minister Walid Muallem.

Zarif met Russian Foreign Minister Sergei Lavrov before the three diplomats were due to try to come up with a joint stance that would keep President Bashar al-Assad in power when the Syrian peace talks begin in Switzerland on Wednesday.

"This does not mean that we have some tri-party (peace) draft," Lavrov told reporters at a joint press appearance with Zarif.

"We have nothing to hide," said Lavrov. "We have no hidden agenda."

Zarif was also to meet Russian President Vladimir Putin later on Thursday to discuss curbs on Tehran's nuclear programme and the possible purchase of missiles that could fend off airstrikes by its arch-foe Israel.

The whirlwind diplomacy in Moscow comes four days after a "Friends of Syria" meeting in Paris of mainly Western and Gulf nations backing the rebels.

"There is a strong Tehran-Moscow-Damascus axis emerging," said Russian PIR Centre research institute analyst Andrei Baklitsky.

"Russia and Iran support Assad and a political settlement to the conflict -- and this is the only thing working right now," said the analyst.

"The West has no other alternative."

Putin has been on the ascendant ever since managing to avert seemingly inevitable US strikes against Russia's closest Middle East ally in September by forcing Assad to renounce his chemical arms.

Now Moscow wants to convince Washington to accept Tehran's presence at the so-called Geneva II conference in order to bolster its efforts to keep Assad in power and curb the future influence of his foes.

"We expect (Geneva II) to include all parties that are capable of making a positive contribution to settling the conflict," Putin told a Kremlin awards ceremony prior to his meeting with the Iranian diplomat.

Zarif said only that Iran would attend the Swiss meetings "if we are invited".

But the Russian foreign ministry also stressed in a statement that the talks should be "based on the provisions of the (June 2012) Geneva Communique" -- a document Iran rejected because it paved the way for a transitional government that could potentially replace Assad.

The United States says Iran must sign up to the accord before it can formally join the talks.

Iran has rejected the condition and UN Secretary General Ban Ki-moon said on Wednesday that no final decision had been reached on the Islamic republic's involvement.

Russian missile purchases

Analysts said Moscow and Tehran are now trying to draw up their own post-war plan that is based on Washington's growing anxiety about the presence of Al-Qaeda sympathisers in rebel ranks.

"A large part of these negotiations are focused on what happens after Geneva II," said Alexander Konovalov of Moscow's Institute for Strategic Assessment.

The Kremlin appears to hold strong leverage over the Islamic republic because of Iran's desire to purchase Russian missiles and other high-tech arms.

This can help Moscow wrest concessions sought by the West over Tehran's support for Hezbollah forces backing up Assad from Lebanon as well as its contested nuclear drive.

Iran has agreed to curb its nuclear programme starting January 20 in exchange for about $7 billion in sanctions relief.

Moscow in 2010 bowed to US and Israeli pressure by aborting an $800-million deal to supply Tehran with an S-300 surface-to-air missile system that would have imperilled any Israeli jets targeting Iranian nuclear sites.

But Iran's Fars news agency said a new Tehran delegation will shortly try to purchase missiles that could be worth even more money and include the more powerful Antey-2500 system.

Russia's Kommersant daily said the talks with Zarif will also include a discussion of Russia's possible oil purchases from Iran in return for equipment and goods that are in short supply because of the current sanctions regime.

A Russian government source told the paper that Moscow was free to sign such a deal because it "never agreed to Western sanctions".

A top US official said Secretary John Kerry expressed his "serious concern" about the reported negotiations during talks with Lavrov in Paris on Monday.

"If the reports are true, such a deal would raise serious concerns... and could potentially trigger US sanctions," White House National Security Council spokeswoman Caitlin Hayden told reporters.