One of Assad's biggest enemies, France, was also concerned that hundreds of fighters linked to attacks in Paris and Brussels might escape. The French have contributed ground and air support to the Mosul campaign.
A week after the campaign was launched, French President Francois Hollande said any flow of people out of Mosul would include "terrorists who will try to go further, to Raqqa in particular".
Still, the battle plan did not foresee closing the road to the west of Mosul until Prime Minister Haider al-Abadi agreed in late October to despatch the Popular Mobilisation militias.
"The government agreed to Iran's request, thinking that it would take a long time for the Hashid to get to the road to Syria, and during that time the escape route would be open and the battle would still proceed as planned," Hashemi said.
The Hashid move to cut the western corridor was announced on Oct. 28, 11 days after the start of the wider Mosul campaign. Fighters made swift progress, sweeping up from a base south of Mosul to seal off the western route out of the city.
Abadi "was surprised to see them reaching the road in just a few days," Hashemi said. "The battle has taken a different shape since then - no food, no fuel is reaching Mosul and Daesh (Islamic State) fighters are bent on fighting to the end.""