The Iraqi air force dropped leaflets on Friday urging residents in the Islamic State-held Old City center of Mosul to flee, raising fears among humanitarian organizations for the safety of civilians there, Reuters reported.
The leaflet drop, announced in an Iraqi military statement, signaled that the decisive offensive to dislodge the militants from their remaining enclave in the northern Iraqi city was imminent.
The U.S.-backed offensive on Mosul, now in its eighth month, has taken longer than planned as the militants are dug in among civilians, fighting back with booby traps, suicide cars and motor-bikes, snipers and mortar fire.
Civilians trapped behind Islamic State lines face a harrowing situation with little food and water, no electricity and limited access to hospitals.
"Iraqi air force planes dropped hundreds of thousands of leaflets a short while ago on the non-liberated areas ... urging citizens to exit through safe corridors," an Iraqi military statement said.
The humanitarian group Oxfam said the leaflets suggested that the Iraqi forces' move on the Old City in Mosul "is imminent ... This could involve an official announcement from the military in the coming days".
"Save the Children is deeply concerned that any calls to leave west Mosul will mean that civilians, particularly children, are in significant danger of being caught in the crossfire," another aid organization said in a statement.
The United Nations last week said up to 200,000 more people could flee Mosul as fighting moves to the Old City.
About 700,000, about a third of the pre-war city's population, have already fled, seeking refuge either with friends and relatives or in camps.
Prime Minister Haider al-Abadi had initially hoped Mosul would be retaken by the end of 2016.
The fall of Mosul would mark the end of Iraqi half of the "caliphate" declared nearly three years ago by Islamic State leader Abu Bakr al-Baghdadi, which also covers parts of Syria.
Iraqi military commanders had expressed hope of taking control of the Old City's Grand al-Nuri mosque, from which Baghdadi declared the caliphate, before the fasting month of Ramadan, which starts on Saturday or Sunday in Iraq.
The insurgency is expected to continue in the sparsely populated desert region along the Syrian border even if Mosul is fully captured.
Iranian-backed Shi'ite paramilitary forces are fighting Islamic State in that part of the country where Baghdadi is believed to be hiding, according to U.S. and Iraqi officials.
The Iraqi Shi'ite paramilitary force, known as Popular Mobilisation, is theoretically under Abadi's command.
On Friday, it announced it had captured the Sinjar military base, cutting the road between two cities that remain under Islamic State control west of Mosul, Tal Afar and Baaj, and getting closer to the Syrian border.
Iraq's Shi'ite-led government is aiming to control the border in coordination with the Iranian-backed army of Syrian President Bashar al-Assad.
Linking up the two sides would give Assad a significant advantage in fighting the six-year rebellion against his rule.