Iran has been emboldened by Washington's decision to hold fire on launching military strikes against its key ally Syria, which has also boosted Tehran's position on its controversial nuclear drive, experts say.

Iran's conservative press trumpeted as a "humiliating blow" the decision by its arch-foe the United States to put on hold plans for a military intervention in Syria following a surprise Russian initiative aimed at defusing the stand-off.

Analysts were more cautious but did agree that Iran stood to benefit from the decision.

"By taking the Russian initiative seriously, Washington will send a signal to Tehran that it's willing to take risks and make compromises for peace," Reza Marashi of the National Iranian American Council, a Washington-based advocacy organisation, told AFP.

Moscow's initiative came as US President Barack Obama was seeking congressional approval to launch punitive strikes against the Damascus regime for allegedly using deadly sarin gas against its own people last month.

Amir Mohebian, a Tehran-based conservative commentator, says Obama's decision to put possible military action on hold "strengthens Iran's position in the region".

Tehran has provided Damascus with material and intelligence support, but denies accusations that it has armed the Assad regime to fight the uprising-turned-civil war that has claimed more than 110,000 lives since 2011.

"Perhaps the Americans now understand that it is less costly to solve the Syrian crisis with Iran's help," Mohebian told AFP.

"It may also affect the issue of (Iran's) nuclear programme" -- which Western powers and Israel suspect has military objectives despite Iranian denials.

Iran had from the outset opposed the US threats to strike Syria, with Foreign Minister Mohammad Javad Zarif very publicly urging many of his Arab, Asian, Latin American and European counterparts to do likewise.

At the same time Iran -- itself a victim of deadly gas attacks during its 1980s war with Iraq -- denounced the use of chemical weapons.

But it also echoed claims made in Damascus and Moscow, another key Assad ally, that rebels, not the Syrian government, had carried out the August 21 attacks that killed hundreds of people.

A Western diplomat in Tehran, speaking on condition of anonymity, said the Iranian leadership, who had feared a military escalation in the region, was now "relieved".

"The Russian proposal has given Iran more room" over Syria, the source said.

The US, France and Israel have maintained that any action against Syria would also send a message to Iran, which is embroiled in a decade-long stand-off with world powers on its nuclear ambitions.

US Secretary of State John Kerry warned last week that inaction on Syria would embolden Iran, while Israeli Prime Minister Benjamin Netanyahu said on Wednesday that Syria must be stripped of its chemical weapons as a lesson to the Islamic republic, the Jewish state's long-time foe.

The election of moderate Hassan Rowhani as Iran's president in June raised hope in the West that Tehran would engage constructively with the international community on its nuclear drive.

Marashi argues that if the White House had gone ahead with attacking Syria, it would have reduced "the manoeuvrability that Rowhani and Obama need as they work to untangle US-Iran relations" -- which observers say is key to solving the nuclear issue.

Rowhani has said he would not shy away from negotiations if it were in Iran's interest, signalling that he could be open to direct talks with Washington with whom Tehran has had no diplomatic relations for more than three decades.

The final decision on such negotiations, or any change to the nuclear programme, however, would have to come from Iran's ultimate authority, supreme leader Ayatollah Ali Khamenei.

Rowhani on Tuesday insisted that Iran would not give up "one iota" of what it considers its right to develop a nuclear programme for peaceful purposes.

This strong stand, which echoes the position of his hardline predecessor Mahmud Ahmadinejad, comes as world powers prepare to restart negotiations with Tehran over its nuclear programme.