At least 284 members of Iraqi security forces have been killed since the battle to retake western Mosul began, and more than 1600 have been wounded, the US military reports.
In addition to the casualties in western Mosul, the battle for eastern Mosul left 490 Iraqi security force members dead, and over 3000 wounded, Army General and head of the US military's Central Command, Joseph Votel, said in a congressional hearing on Thursday (local time).
Iraqi special forces and police fought Islamic State militants to encroach on the al-Nuri mosque in western Mosul on Thursday.
The close-quarters fighting is focused on the Old City surrounding the mosque, where Islamic State leader Abu Bakr al-Baghdadi proclaimed a caliphate nearly three years ago across territory controlled by the group in both Iraq and Syria.
Thousands of residents have fled from IS-held areas inside Mosul, but tens of thousands are still trapped inside homes.
With the battle entering the densely populated areas of western Mosul, civilian casualties are becoming more of a risk.
The United Nations says several hundred civilians have been killed in the last month, and residents say IS militants are using them as human shields.
Lieutenant General Stephen Townsend said increases in casualties were expected as the war entered its deadliest phase.
Local officials and eyewitnesses reported that 240 people may have been killed in the Al-Jadida district when a huge blast caused a building to collapse.