Shia fighters take main route linking Tal Afar to Sinjar, blocking ISIL’s crucial supply route from Mosul into Raqqa.
ISIL’s last supply line from Mosul to Syria has been severed by Iraqi-led forces, leaving the armed group’s stronghold completely isolated.
Shia-Muslim paramilitary forces, known as Hashed al-Shaabi, captured the road linking Tal Afar to Sinjar west of Mosul on Wednesday and linked up with Kurdish forces there, security officials say.
"Hashed forces have cut off the Tal Afar-Sinjar road," Abu Mahdi al-Mohandis, a senior commander, said on social media.
A Kurdish security official told AFP news agency the Shia militia had linked up with other anti-ISIL forces, including Kurdistan Workers’ Party (PKK) fighters, in three villages in the area.
Also on Wednesday, an air strike by the US-led coalition "disabled" the fourth bridge on the Tigris River in Mosul, leaving the city with a single functioning connection and further disrupting ISIL’s supply lines.
Aamaq news agency, ISIL’s media arm, and a top Iraqi commander in Mosul reported the air strike, which took place before dawn.
It was the second to target a Mosul bridge this week and the fourth since shortly before the launch of the offensive to retake the city.
Iraqi forces kicked off the operation - backed by US-led air strikes - on October 17 to retake the country’s second-largest city, where ISIL leader Abu Bakr al-Baghdadi proclaimed a caliphate in 2014.
Troops have already entered the city from the east, Kurdish Peshmerga and other forces are also closing in from the north and south and only the west had remained open.
The latest development will make it long and dangerous for ISIL if it attempts to move fighters and equipment between Mosul and the Syrian city of Raqqa, the last two bastions of their crumbling "caliphate".
Tens of thousands of Iraqi civilians have fled Tal Afar as the paramilitary force closed in on the town.
The exodus from Tal Afar, 60km west of Mosul, is causing concern among humanitarian organisations as some of the fleeing civilians are heading deeper into ISIL territory, where aid cannot be sent to them, provincial officials said.
About 3,000 families have left the town, with about half heading southwest towards Syria, and half northward into Kurdish-held territory, said Nuraldin Qablan, a Tal Afar representative in the Nineveh provincial council, now based in the Kurdish capital, Erbil.
"We ask Kurdish authorities to open a safe passage for them," he told Reuters news agency. "People are fleeing due to the Hashid’s advance, there are great fears among the civilians."
Iraqi military estimates put the number of ISIL fighters in Mosul at 5,000-6,000, facing a 100,000-strong coalition of Iraqi government units, Peshmerga fighters, and Shia militias.
Mosul’s capture is seen as crucial towards dismantling the caliphate. Baghdadi is believed to have withdrawn to a remote area near the Syrian border, and told ISIL fighters there can be no retreat.
A Mosul resident said air strikes have intensified on the western part of the city, which is divided by the Tigris River running through its centre.
The strikes targeted an industrial area where ISIL is believed to be making booby-traps and transforming vehicles into car bombs, he said.
ISIL fighters are dug in among more than a million civilians as a defence tactic to hamper the air strikes. They are moving around the city through tunnels, ploughing suicide vehicle bombs into advancing troops, and hitting them with sniper and mortar fire.