Six weeks into the offensive on Mosul, Daesh is fighting in the area of Tal Afar, 60 km (40 miles) to the west, against a coalition of groups known as Popular Mobilization Units.
Cutting the western road to Tal Afar would seal off Mosul as the city is already surrounded to the north, south and east by Iraqi government and Kurdish Peshmerga forces.
The government's Counter Terrorism Service unit breached Daesh's defenses at the end of October and is fighting to expand a foothold it gained on the eastern side of Mosul.
The road to Tal Afar is no longer safe, said a truck driver who used it two days ago to bring in fruit and vegetables from Raqqa, Daesh's Syrian stronghold.
He said he saw three trucks burning on the road while fighting raged in the vicinity. "This is the last time I drive on this road, it will be cut," he told Reuters by telephone, asking not to be identified as the insurgents punish by death those caught communicating with the outside world.
A Popular Mobilization spokesman said over the weekend that its forces were already advancing to the main highway as part of operations to seal off Mosul. A Reuters reporter said they were massing troops to finish encircling Tal Afar.
The Iraqi airforce supporting Popular Mobilization in their fight near Tal Afar carried out airstrikes that killed 15 insurgents, including a group that was hiding in a tunnel near Tal Afar airbase, according to a military statement published Sunday evening.