Hashed al-Shaabi (Popular Mobilisation) paramilitary forces reached the road linking Tal Afar to Sinjar, west of Mosul, and linked up with Kurdish forces there, the officials said.
"Hashed forces have cut off the Tal Afar-Sinjar road," senior Hashed commander Abu Mahdi al-Mohandis said on social media, referring to two towns on the road linking Mosul to Syria.
A Kurdish security official told AFP that Hashed forces had linked up with other anti-IS forces, including Kurdistan Workers' Party (PKK) fighters, in three villages in the area.
Iraqi forces launched a major offensive on October 17 to retake Mosul, which is the country's second city and where jihadist supremo Abu Bakr al-Baghdadi proclaimed a caliphate in 2014.
Federal forces 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 very long and dangerous for IS if it attempts to move fighters and equipment between Mosul and the Syrian city of Raqa, the last two bastions of their crumbling "caliphate".