Hundreds of Iran-backed Iraqi fighters crossed into Syria on Monday to help the government fight rebels who seized Aleppo last week, but Lebanon's Hezbollah has no plans for now to join them, according to sources.

Iran's constellation of allied regional militia groups, aided by Russian air power, has been integral to the success of pro-government forces in subduing rebels in Syria who rose up against President Bashar al-Assad in 2011. But that alliance faces a new test after last week's lightning advance by rebels in northwest Syria, with Russia focused on war in Ukraine and Hezbollah's leadership decimated by a war with Israel that ended in a ceasefire last week.

The rebel storm of Aleppo is the biggest success of anti-Assad fighters for years. Government forces had held complete control of Aleppo since capturing what was then Syria's largest city in a siege in 2016, one of the major turning points of a war that has killed hundreds of thousands of people. The head of Syria's main opposition group abroad, Hadi al-Bahra, told reporters the rebels were able to seize the city so quickly because Hezbollah and other Iran-backed groups were distracted by their conflict with Israel.