DUBAI (Reuters) - A top adviser 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.

Assad's fate is a sticking point in talks between world powers aimed at finding a political solution to the crisis in Syria. Iran and Russia want him to stay in power until elections are held, while Western and Arab powers say he must go.

"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 adviser 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.

Velayati also said Iran would try to ease tensions between Turkey and Russia. Turkey shot down a Russian fighter jet last month that it said had violated Turkish airspace while flying a mission in Syria.

"There is no benefit to tensions mounting up in the region. We must not take the side of either party, and have a duty to reduce tensions between these two countries," he said.