Khamenei was quoted as saying by IRNA news agency: “There should be no doubt that what happened in Syria was the result of a joint American-Zionist plot.”

“Yes, a neighbouring state of Syria clearly played a role in this matter and continues to do so — everyone can see that,” he said but did not name the nation.

He said: "The main conspirator, the main planner, and the command centre lie in America and the Zionist regime.”

The toppling of Assad's 24-year-old regime is a blow to the Iran-led 'Axis of Resistance' political and military alliance.

The alliance mostly opposes Israeli and US influence in the Middle East.

“I tell you that By God’s grace, Iran is strong and powerful and will become more powerful,”  Ayatollah Ali Khamenei said.

Meanwhile, rebels have taken control over the Syrian city of Deir Ezzor, just days after they seized the capital Damascus, leading to the fall of ex-President Bashar al-Assad's regime.

“We announce the city of Deir Ezzor and its military airport as completely liberated,” Military Operations Command spokesperson Hassan Abdul Ghani was quoted as saying by CNN.

An SDF source told CNN that the Kurdish forces have withdrawn only from the western bank of the Euphrates in Deir Ezzor city.