The fall of Syrian President Bashar al-Assad underscores Russia's weakness and inability to fight on two fronts, Ukraine's foreign ministry said on Sunday.

Russia had bolstered Assad's government by staging air strikes against opposition targets beginning in 2015 and had operated out of two bases on Syrian territory.

But Moscow's 33-month-old invasion of Ukraine has sapped considerable military resources.

"Events in Syria demonstrate the weakness of Putin's regime, which is incapable of fighting on two fronts and abandons its closest allies for the sake of continued aggression against Ukraine," the foreign ministry said in a statement.

Russia said earlier that Syrian President Bashar al-Assad had left office and departed his country after giving orders for a peaceful transfer of power, but did not say where he was now or whether the Russian military planned to stay in Syria.

The HUR intelligence directorate, posting on the Telegram messaging app, said Russian forces in Syria "had withdrawn its warships from the naval base in Tartous which Assad had allowed Moscow to use as payment for his security."

A frigate, the Admiral Grigorovich, as well as a cargo ship, the Engineer Trubin, were pulled out of Tartous on Sunday, it said and Russian aircraft were moving "the remnants of their weapons and military equipment" from Khmemim air base.

HUR provided no evidence of its affirmations and Reuters was not immediately able to verify the situation.

Russian war bloggers warned at the weekend that the two bases and Moscow's very presence in the Middle East were under threat from the insurgents.