The International Committee of the Red Cross has also issued a statement warning of a mass flight from Mosul, although without giving an estimate of the number likely to be affected. “The broadening of the conflict to populated areas along the Tigris and Euphrates rivers will create more humanitarian needs,” the ICRC warns. “If major cities such as Mosul come under fire again, thousands more people will have to flee.”

Syed Jaffer Hussain, the World Health Organisation’s representative in Iraq, also says that an attempt to recapture Mosul could lead to hundreds of thousands seeking refuge in Kurdistan. The exodus could begin as soon as Isis becomes unable to stop people leaving Mosul, and the US increases the number of its air strikes.

Scepticism has been expressed about the ability of the Iraqi army to recapture the city, but even the beginning of an attempt to do so might lead to a mass flight. The US Central Command said earlier this month that an offensive to retake the city would start in April or May and would involve up to 25,000 Iraqi soldiers, although the exact date would depend on their degree of combat readiness.

The WFP said it would have “to pre-position stocks of food, which we are doing now”. These stockpiles will be held in the three main cities of the Kurdistan Regional Government (KRG) – Irbil, Duhok and Sulaimaniyah – and also in Kirkuk which is under Kurdish control but outside the KRG.

One problem is that Sunni Arabs from Mosul are currently banned from entering the KRG, which has already given refuge to 1.4 million people displaced by fighting in Iraq and Syria. Aid agencies are hopeful that, in the event of an exodus from Mosul, the KRG authorities will change their mind and let them into its territory. If they do not, those in flight from Mosul would probably head for Kirkuk. Before an expected IS drive towards Kirkuk at the end of January, one road was opened between it and Mosul. The WFP is already feeding 80,000 people in the Kirkuk area.

The Sunni Arab population of Mosul has strong motives for trying to escape any battle for their city. The Shia-dominated Iraqi army held Mosul for 10 years up until 2014, during which time they acted very much as a foreign occupation force widely resented by Sunnis. The Isis victory and the Iraqi army’s disintegration was widely welcomed by them.

They are also fearful that the notoriously sectarian Shia militia forces, which number some 120,000 men, would be involved in any assault on Mosul. Where they have captured Sunni towns and villages around Baghdad in the past, they have treated all those who have not fled as Isis sympathisers, regardless of their actual allegiance, if any. Young Sunni men have been detained, tortured, held for ransom or killed. Sunni residents of Mosul suspect that the same thing could happen to them.

Even if Mosul did not fall to the Iraqi army, Kurdish Peshmerga or other anti-Isis forces, an attempt to capture it would involve heavy US air strikes. During the four-month siege of the Syrian Kurdish town of Kobani, much of the city was destroyed by US bombs aimed at Isis militants. Aside from civilian casualties, an air assault would further reduce Mosul’s already limited supplies of electricity, fuel and clean water. Many people are in hospital suffering from intestinal illnesses brought about by drinking dirty water.

The WFP’s Marwa Awad said any exodus from Mosul would be the latest in a series that has uprooted 2.2 million people from their homes since January 2014. It was in that month that Isis captured the city of Fallujah, 40 miles west of Baghdad, and began a long struggle with the Iraqi army for the vast Sunni Arab province of Anbar that sprawls across the western third of Iraq. The jihadis now hold 85 per cent of the province.

It was this war in Anbar, in the first half of 2014, that provoked the first wave of 450,000 refugees seeking safety elsewhere in Iraq. In June, the capture by Isis of Mosul and much of northern Iraq outside Kurdish-held areas created a further 500,000 refugees. An Isis offensive directed against the Kurds in August displaced another 600,000, many of them Yazidis from Sinjar, west of Mosul, who were terrified by the threat of massacre, rape and slavery.

Christians, who had been forced out of Mosul city in June and July, saw their towns and villages in the Nineveh Plain around Mosul seized, forcing them to flee into Kurdistan. Since August, a further 650,000 have been displaced by fighting, mostly in the provinces around Baghdad.

It is extremely unlikely that Isis would give up Mosul without a fight to the death, because it was the city’s unexpected capture by the extreme jihadi movement last year that enabled it to proclaim the caliphate on 29 June. Its loss would constitute a devastating blow to its prestige and sense that its victories are divinely inspired. Even at Kobani, where Isis forces were in a poor tactical positions battling Syrian Kurdish fighters known for their determination and discipline, Isis was able to hold on for three months despite presenting an ideal target for 700 US air strikes.

While all Iraqi communities have been forced to flee at one time or another, the Sunni have nowhere safe to go to. Iraqi government and US policy since last June had been to divide the Sunni community and turn part of it against Isis, as the US succeeded in doing in 2006-07. But Isis mercilessly punishes any Sunni whom it suspects of working with its opponents. As a result there have been very few signs of overt resistance to Isis in Mosul or elsewhere in Isis-occupied territory. One Kurdish observer, who did not want to be named, said: “We don’t see daily assassinations and bombings which used to happen when the Americans and later the Iraqi army were running Mosul. Sunni leaders outside the city exaggerate or even invent their support.”

The savagery of the sectarian and ethnic conflict in Iraq is now such that no community feels safe under the rule of another, preferring to take to the roads in a bid to survive.