Iran would consider any attack on Syria an attack on itself, a senior government official was quoted as saying on Saturday, in one of Tehran's most assertive defences of its ally yet.
Iran is a key supporter of Syrian President Bashar al-Assad who is fighting a near two-year-long revolt. Tehran has already repeatedly warned the West against intervening in the conflict against Assad.
"Syria has a very basic and key role in the region for promoting firm policies of resistance ... For this reason an attack on Syria would be considered an attack on Iran and Iran's allies," said Ali Akbar Velayati, an aide to Supreme Leader Ayatollah Ali Khamenei, according to the Mehr news agency.
Tehran sees Damascus as part of an axis of opposition to Israeli and Western influence in the Middle East.
In September, an Iranian military official was quoted as saying Iran would take action if the United States was to attack Syria.
Iran and Syria, whose leader follows an offshoot of Shiite Islam, signed a mutual defence pact in 2006, but little is known of its details or whether there are any other signatories.
Iran accuses Western powers and regional states of supporting and arming the Syrian rebels, while the rebels accuse Iran of sending fighters from its Islamic Revolutionary Guard Corps to help Assad crush the uprising.