Iraqi forces are reported to have taken control of the last major road out of western Mosul, preventing Islamic State militants from fleeing the city.
A source in the paramilitary Popular Mobilisation force told the BBC that the highway was cut near the village of Badush, about 10km (six miles) away.
The route leads to Tal Afar, another IS stronghold that is 40km further west.
Government forces already control the east of Mosul and they began an assault aimed at capturing the west last month.
They have since driven militants from the international airport, a military base, a power station and a number of residential areas, according to the military.
The source in the Popular Mobilisation, which is dominated by Iranian-backed Shia militias, said its fighters and soldiers from the Iraqi army's 9th Armoured Division had taken control of the highway to Tal Afar near Badush on Tuesday night.
A general in the 9th Armoured Division separately told Reuters news agency that other units were within 1km of the north-western entrance to Mosul.
"We effectively control the road, it is in our sight," he said.
Residents had confirmed they could no longer use the highway, Reuters reported.
In November, Popular Mobilisation fighters advancing from the south cut the road west of Tal Afar, effectively encircling IS-held territory around Mosul.
They then started working to block the route between Tal Afar and Mosul to prevent IS from using it to send reinforcements and supplies to the city.
Inside western Mosul on Wednesday, officers from the federal police Rapid Response Force were said to be approaching the city's government buildings.
The force's commander, Maj-Gen Thamir al-Husseini, told the Associated Press that they were about 800m (2,600ft) from the complex.
Although they are located near the old city, recapturing the buildings would be a largely symbolic victory.
On Monday, the Rapid Response Force reached the western end of a bridge over the River Tigris, giving the government control of a crossing for the first time since the Mosul offensive began in October.
All of Mosul's five bridges have been badly damaged in fighting since October. But once repaired, they could help the military bring in reinforcements and supplies from the government-held east.
Meanwhile, the British deputy commander of the US-led coalition military operation against IS has predicted a "grinding" fight to retake the rest of Mosul.
Maj-Gen Rupert Jones said around 100,000 buildings would have to be cleared in what was expected to be house-to-house fighting.
Speaking to journalists on a visit to London, Gen Jones said it would be "tough and take time" but that he was confident the Iraqi forces would complete the job.
He described the use of drones by IS to drop bombs on troops in Mosul as an "insidious threat" but not "a game-changer", adding that the coalition was working to defeat the threat, he added.