Isis fighters are now trapped in Mosul as Iraqi forces cut off the last road out of the city, the US envoy to the anti-Isis coalition said. "Any of the fighters who are left in Mosul, they're going to die there."
The jihadists are vastly outnumbered and outgunned by Iraqi forces, backed by a US-coalition, and are defending their last major stronghold in Iraq using suicide car bombs, snipers and mortars.
However, fight is expected to become tougher as soldiers push into the Bab al-Tob area of the Old City, due to narrow alleyways through which armoured vehicles cannot pass.
As many as 600,000 civilians are trapped with the militants inside the city which Iraqi forces have effectively sealed off from the rest of the territory Isis controls in Syria and Iraq.
Major General Maan al-Saadi said the militants were showing signs of weakness despite their initial "fierce" resistance.
"The enemy has lost its fighting power and its resolve has weakened. It has begun to lose command and control," he said, adding that around 17 out of 40 western districts had been retaken.
Mr Saadi said he expected it would take less time to recapture the western half of the city than the east, which was cleared in January after 100 days of fighting.
Isis released dozens of prisoners held in jails in a sign the militants are being overwhelmed by the Iraqi offensive, launched three weeks ago.
Among those released were people who had been caught selling cigarettes, violating a smoking ban, or in possession of a mobile phone and therefore suspected of communicating with the outside world, local residents said.
It came as remains of hundreds of mainly Shia prison inmates killed by Isis were unearthed by forces retaking the Badush area.
"Initial checks of part of the mass grave revealed remains with prison uniforms and lined up in a way that indicates they were shot dead in groups," said Karim Nouri, spokesman for the Hashid Shaabi, a state-run umbrella for Shia paramilitary groups.
Human Rights Watch said in a report that as many as 600 people were killed in the Badush prison massacre, which took place on the same day Isis militants captured Mosul in June 2014.
More than 65,000 people have been displaced by the fighting in the past two weeks alone, bringing the total number to more than 200,000 since the campaign to recapture Mosul began, according to the International Organisation for Migration.
It is by far the largest city Isis has held since the group's leader, Abu Bakr al-Baghdadi, proclaimed a caliphate spanning Iraq and Syria from a mosque in Mosul in the summer of 2014.
The group has been losing ground in both countries to an array of forces, some of which are backed by the United States, others by Turkey, Russia and Iran.
Losing Mosul would be a major blow to Isis, but the group is expected to pose a continuing threat, reverting to insurgent tactics such as bombings.
Additional reporting by agencies