Turkish Foreign Minister Mevlut Cavusoglu called for an immediate ceasefire in Syria on Friday, describing the situation in Aleppo as critical and saying that President Bashar al-Assad was unfit to rule.

Asked about Assad at a news conference in Beirut, Cavusoglu said it was undeniable that the Syrian leader was responsible for 600,000 deaths and that somebody with that record should not be running a country.

NATO member Turkey is a major backer of rebels fighting to oust Assad. The rebels have come under siege in eastern Aleppo after rapid advances by Syrian government forces in recent days, bringing them to the brink of a major defeat.

Cavusoglu said Turkey was speaking with Assad's allies Russia and Iran, as well as with Syria and Lebanon, about trying to find a solution in Syria.

Turkish President Tayyip Erdogan and Russia's Vladimir Putin have spoken by phone at least three times over the past week about Syria, while Russian Foreign Minister Sergei Lavrov met Cavusoglu in Turkey on Thursday.

Cavusoglu said after that meeting that Moscow agreed on the need for a halt to fighting and the provision of aid in Aleppo, but deep divisions remain between them over the conflict.