* Mosul fall would effectively end Iraqi half of IS's 'caliphate'
* Humanitarian groups express concern for civilians' safety
* People eating plants as food prices increase 10 fold
* Iraqi planes drop leaflets calling on civilians to flee
* Old City makes up most remaining IS-held enclave in Mosul
Iraqi armed forces launched an operation on Saturday to capture the last Islamic State-held enclave in Mosul, according to a military statement.
The fall of the city would effectively 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.
The Iraqi air force dropped leaflets on Friday urging residents in the enclave to flee, although humanitarian groups fear for the safety of civilians trying to escape.
The enclave covers mainly the Old City centre and three adjacent districts alongside the western bank of the Tigris river.
The US-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.
"The joint forces have began liberating the remaining districts," said an Iraqi military statement.
Desperate civilians trapped behind Islamic State lines now face a harrowing situation with little food and water, no electricity and limited access to hospitals.
The push inside the Old City coincides with the start of the holy fasting month of Ramadan. Its prime targets is the medieval Grand al-Nuri mosque and its landmark leaning minaret where Islamic State's black flag has been flying since mid 2014.
The forces hope to capture in the next few days the mosque where Baghdadi revealed himself to the world and announced the "caliphate" that also spans part of Syria.
Residents in the Old City sounded desperate in telephone interviews made over the past few days.
"We're waiting for death at any moment, either by bombing or starving," one said, asking not to be identified for his own safety. "Adults eat one meal a day, either flour or lentil soup."
The United Nations expressed "deep concern" for the hundreds of thousands of civilians trapped behind Islamic State lines, in a statement on Saturday from the organisation's under-secretary general for humanitarian affairs, Stephen O'Brien.
"Although the UN is not present in the areas where fighting is occurring, we have received very disturbing reports of families being shut inside booby-trapped homes and of children being deliberately targeted by snipers," he said.
The militants have laid sheets of corrugated metal over pebbles in the alleys as an early warning system, residents said. The grinding noise produced by treading on it would alert them to any troop movements or civilians trying to escape.
The United Nations last week said up to 200,000 more people could flee Mosul as fighting moves to the Old City.
Residents said millet, usually used as bird feed, is being baked like rice as food prices increased ten fold. People were seen collecting wild mallow plants in abandoned lots and also eating mulberry leaves and other types of plants.
About 700,000 people, about a third of the pre-war city's population, have already fled, seeking refuge either with friends and relatives or in camps.
Iraqi Prime Minister Haider al-Abadi had initially hoped Mosul would be retaken by the end of 2016.
The insurgents are also retreating in Syria, mainly in the face US-backed Kurdish-led forces.
The insurgency is expected to continue in the sparsely populated desert region along the Syrian border even if Mosul is fully captured.
Iranian-backed Shia paramilitary forces are fighting Islamic State in that part of the country where Baghdadi is believed to be hiding, according to US and Iraqi officials.