Fork has appeared in Iran's road to Damascus

Iran's ability to project its power in the Arab world would be greatly diminished if Syria's president, Bashar al-Assad, is replaced by a less-friendly, Sunni-dominated government.

Tehran is relying on him to crush the peaceful unrest in the same way that Iran suppressed the mass pro-democracy protests ignited by president Ahmadinejad's disputed election in 2009.

Tehran has until now mostly welcomed the changes in the emerging Middle East, viewing them as blows to the US and a boost to its own ambitions.

Iran's most significant diplomatic gain is the apparent readiness of Egypt's new leadership to restore ties with Tehran after 31 years.

However, the picture could change dramatically if the Assad regime falls.

Syria has been the gateway for Iran to extend its reach into the Middle East and a vital connection to Lebanon's Iranian-backed Hizbollah movement, which gives Tehran a proxy presence on Israel's northern border.

Syria and Iran also support the militant Palestinian group Hamas in the Gaza Strip. Tehran has billions of dollars worth of investments in Syria.

Tehran and Damascus make unlikely bedfellows. Non-Arab Iran is a hardline Shi'ite Islamic republic. Syria is a secular Arab state where the majority of the population is Sunni, although Assad's family and the bedrock of his regime are from the minority Alawite sect, an offshoot of Shi'ite Islam.

Even if Assad is toppled, few can predict the shape of any new Syrian government. So it cannot be assumed that his successors would relinquish Syria's alliance with Iran.

But Tehran, allegedly, is taking no chances. Washington says there is "credible information" that Iran is helping Syria to quell protests.

The accusation followed a Wall Street Journal report that Iran has provided Damascus with crowd control equipment and technical assistance to monitor communications used by opposition groups.

Tehran and Damascus have denied the charges.