The few thousand militants still fighting in southern Mosul are outnumbered by a 100,000-strong array of Iraqi forces.
A senior official of the US-led coalition in Iraq said Sunday that the remaining western Mosul neighborhoods held by the Islamic State have been surrounded and the Iraqi army has seized control of the last road leading out of the city.
Iraqi forces are fighting the Islamic State in western Mosul after declaring the city's east "fully liberated" in January.
A fighter from Iraq's elite Counter-Terrorism Service takes cover inside a building in Shuhada, west Mosul.
An Iraqi Shia paramilitary group, the Iran-backed Popular Mobilisation Forces (PMF) of Shia jihadists, claimed to have made the discovery after driving the militants from the Badush area where the prison is located, as part of a wider United States and Iran-backed campaign to dislodge Daesh from the city of Mosul.
Jonah's mosque was blown up in July 2014, but experts surveying the damage after it was recaptured in January by a US -backed Iraqi campaign found a network of tunnels dug by the militants, leading down to a 7th century BC Assyrian palace.
Iraqi forces have continued their push into Mosul's old city, an area of narrow streets and alleys.
The IS is alleged to have killed up to 600 inmates, majority Shia Muslims, when it seized Mosul in 2014.
According to Reuters, losing control of Mosul would be a major blow to ISIS.
He said forces had captured 17 out of 40 western districts.
Iraq launched its military offensive to retake western Mosul from the Islamic State on February 19.
Hashed forces found "a large mass grave containing the remains of around 500 civilian prisoners in [Badush] prison who were executed by [ISIL] gangs after they controlled the prison during their occupation of Mosul", the military said. Since then, more than 200,000 residents have fled the city, with 65,000 fleeing in just the past two weeks as fighting there has intensified, according to numbers from the International Organization for Migration.
Coalition forces aided by US-led airstrikes began the operation to retake the city in October.
Almost 100,000 people have been displaced since the operation to liberate western Mosul began last month, according to Iraq's Ministry of Migration and Displaced.
On Wednesday, bomb blasts ripped through a wedding party near Tikrit, which was recaptured by Iraqi forces in 2015, killing more than 20 people.
Federal Police Chief Lieutenant General Raed Shaker Jawdat told Asharq Al-Awsat on Sunday that Iraqi forces have now retaken more than one third of Mosul.