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.

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.

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

What’s happening in Aleppo? What we know about sudden rebel gains in Syria’s 13-year war and why it matters - Russia and Syrian warplanes launch strikes as insurgents seize Aleppo in shock advance

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.

A surprise offensive by rebel forces opposed to Syrian President Bashar al-Assad's regime has led to a new round of fighting in a largely static civil war.