Syria's civil war has taken a dramatic turn, with dictator Bashar Assad once again leaning on Russian support after a shock offensive by Islamist rebels in Aleppo.

Russia's Defence Ministry said its air force had carried out strikes on Syrian rebels in support of the country's army, Russian news agencies reported on Saturday, amid an advance by forces opposed to Syrian President Bashar al-Assad.

Syrian and Russian jets are stepping up strikes on opposition forces in northern Syria in retaliation for the sudden offensive that has cost the regime control of the country’s second largest city, Aleppo.

Hundreds of fighters from Iran-backed Iraqi militias crossed into Syria overnight to help the government fight rebels who seized Aleppo last week, Syrian and Iraqi sources said on Monday, and Tehran pledged to aid the Damascus government.

Iranian-backed Iraqi militias have deployed in Syria to back the government’s counteroffensive against a surprise advance by insurgents who seized the largest city of Aleppo

Approximately 200 Iraqi militiamen crossed into Syria overnight through the strategic Bou Kamal border crossing, according to an opposition war monitor.