Iraqi forces launched a military operation against jihadists on Sunday to retake Mosul's west, Iraqi Prime Minister Haider al-Abadi said, AFP reported.

According to the report, it would be the most brutal fighting yet in a four-month-old operation on the city.

"Our forces are beginning the liberation of the citizens from the terror of Daesh," Haider al-Abadi said in a short televised speech.

"We announce the start of a new phase in the operation. We are coming, Nineveh, to liberate the western side of Mosul," he said.

The jihadists have put up stiff resistance to defend Mosul, their last major stronghold in Iraq and the place where their leader Abu Bakr al-Baghdadi proclaimed a "caliphate" in 2014.

After shaping operations around Mosul, it took Iraq's most seasoned forces -- the elite Counter-Terrorism Service (CTS) -- more than two months to clear the eastern side of Mosul.

While the federal forces' attrition is said to be high, Daesh's had been undoubtedly higher and commanders have said the jihadists may no longer have the resources to defend east Mosul effectively, the report added.