A top advisor to Iran's Supreme Leader on Sunday said the future of Syria's President Bashar al-Assad could only be determined by the Syrian people and this was a "red line" for Tehran.

"Bashar al-Assad is the Islamic Republic of Iran's red line because he was elected president by the Syrian people," said Ali Akbar Velayati, the top foreign policy advisor to Supreme Leader Ayatollah Ali Khamenei.

"The Syrian people must decide their own fate, and nobody outside Syria's borders can choose for the Syrian people," he added.