Bashar al-Assad is in Moscow, Russian state media says

Former Syrian President Bashar al-Assad and his family are in Moscow, according to Russian state media.

Assad fled Damascus on Sunday in the wake of a rebel advance led by Hay'at Tahrir al-Sham.

"Assad and members of his family have arrived in Moscow," a Kremlin source told the TASS and Ria Novosti news agencies.

"Russia granted them asylum on humanitarian grounds."

Russia had been heavily supportive of the Assad government in Syria and their intervention in 2015 was thought to have decisively turned the civil war in Assad's favour after years losing ground.

There are a number of Russian bases in Syria, which made the country a key ally for Moscow in the region.

However, Russia was either unable or unwilling to stymie a lightning takeover of major Syrian cities including Hama, Aleppo and Homs over the past week, which ended with the toppling of Assad's government

Russia said on Sunday it had called for a UN Security Council meeting Monday to discuss the situation in Syria.

"Given the latest events in Syria, of which the depth and consequences for this country and the whole of the region have not yet been measured, Russia has called for urgent closed-door consultations of the Security Council of the United Nations," a senior Russian official attached to the UN posted on Telegram Sunday.

A source speaking to Russian state media also said that the opposition forces had "guaranteed" the security of Russia's bases in Syria.

Rebel fighters entered Damascus around 5am local time without resistance, quickly capturing the international airport, the state TV building and many other strategic government facilities. 

Government forces and personnel reportedly withdrew from their positions, allowing a smooth takeover for the rebels. 

Assad's whereabout were speculated on for hours, with some even suggesting he was on a plane that had left the capital and crashed.