Turkey's top diplomat spoke in Qatar after meetings on Saturday with his Russian and Iranian counterparts on Syria were confounded by the lightning rebel advance which saw the fighters sweep into Damascus.

The "Assad regime collapsed and control of the country is changing hands", Fidan said at the Doha Forum for international political dialogue, adding that Assad was "probably outside of Syria" when asked about his whereabouts.

"This didn't happen overnight. For the last 13 years, the country has been in turmoil," Fidan added, referring to the prolonged civil war which began with Assad's repression of democracy protests in 2011.

Turkey, without being directly involved, has supported various rebel movements and looked favourably on the swift advance through Syria.

Fidan said Ankara had been in contact with rebels in Syria to ensure that the militant Islamic State group and the PKK, the Kurdistan Workers' Party which is banned in Turkey, cannot expand in the security vacuum, "taking advantage of the situation".

Turkey, Iran and Russia have been partners since 2017 in the so-called Astana process seeking to end Syria's civil war that began in 2011.

Moscow and Tehran had both offered military support to help Assad's forces hold off the rebels.

Despite repeated violations, a 2020 ceasefire agreement brokered by Russia and Turkey had largely frozen the conflict for several years, but this was blown apart by the shock offensive launched late last month.

Fidan said international and regional actors should ensure a smooth transition from the Assad government.

He said a new administration should "not pose a threat for its neighbours, on the contrary, the new Syria will address the existing problems, will eliminate the threats".

Turkey shares a 900-kilometre (560-mile) border with Syria and hosts nearly three million Syrian refugees.

Fidan said that following the fall of the Assad government, "millions of Syrians who were forced to leave their homes can return to their land", adding that it was "time to unite and reconstruct the country".