A senior adviser to Iran's parliament speaker on Sunday denied reports about reaching an agreement with Russia on the removal of Bashar al-Assad from power in Syria.

In a tweet, Hossein Amir-Abdollahian described al-Assad as "the legitimate president of Syria."

"Report of a deal between Iran and Russia for his resignation is a big lie and a game by the US-Zionist media," Amir-Abdollahian said.

"Tehran strongly supports the sovereignty, national unity and territorial integrity of Syria," he added.

Iran is a strong ally of al-Assad regime, the forces of which have been fighting opposition groups in a deadly civil war since 2011.

Since then, hundreds of thousands of people are believed to have been killed and millions more displaced by the conflict.