Islamic State militants have closed the streets around Mosul's Grand al-Nuri Mosque, residents said, apparently in preparation for a final showdown in the battle over their last major stronghold in Iraq, Reuters said.

Dozens of fighters were seen by residents taking up positions in the past 48 hours around the medieval mosque, the site where Islamic State leader Abu Bakr al-Baghdadi declared an Islamist caliphate in July 2014.

Islamic State's black flag has been flying from the mosque since the militants captured Mosul and seized swathes of Iraq and Syria in the summer of 2014.

U.S.-backed Iraqi government forces retook eastern Mosul in January and began a new push on Saturday to capture the group's remaining enclave in western Mosul, comprising of the Old City center where the mosque is located, and three adjacent districts alongside the western bank of the River Tigris.

The fall of the city would, in effect, mark the end of the Iraqi half of the self-styled caliphate. Meanwhile in Syria, Kurdish forces backed by U.S.-air strikes are besieging Islamic State forces in the city of Raqqa, the militants' de facto capital in that country.