The ongoing fight to liberate the Isis-controlled city of Mosul is a crucial moment in the war against the Islamic State in Iraq.

The city has become a symbol of Isis power in the region. Mosul was occupied by Isis in June 2014 and Isis leader Abu Bakr al-Baghdadi proclaimed it the crown jewel of the self-declared Islamic State caliphate.

Strategically located along the Tigris river, Mosul sits between routes linking Iraq to Turkey and Syria, bordering the semi-autonomous region of Kurdistan. It is the capital of the Nineveh region, whose oil-rich fields are one of the key resources financing Isis terrorist operations. One of them, the Qayyara oil field, was reconquered at the end of August, but an oil ministry spokesperson told Reuters that production would not be resumed until the security situation improved with liberation.

The citys developed banking system also provided a financial lifeline to the terror group, who was able to loot over 500 billion Iraqi dinars (around £250m) stored in the Central Bank of Mosul, which were left behind by the Iraqi forces.

Isis also took over Mosul University, one of the largest research centres in the Middle East and Iraqs second largest university. It became a basis for Isis fighters and, it was suspected, a centre to develop chemical bombs, which is why it was targeted and destroyed in US air strike in March.

An ethnically-diverse centre, Mosul was inhabited by different people including Assyrian Christians, Yadizi, Turkmen and Kurds, in addition to Sunni Muslim Arabs.

The Yadizi people were particularly targeted by Isis during the occupation. It is estimated that thousands of women and girls are being held captives and subjected to rape and torture. Yazidi activists denounced the lack of a plan to rescue the prisoners as the coalition moves to reconquer the city.

The United Nations warned, in August, that fighting to recapture the city would accelerate the exodus, with the UN refugee agency UNHCR working on building new refugee camps to shelter up to 120,000 refugees. The UN is looking to raise around $284m to face the humanitarian crisis, with less than half raised so far.

The battle for Mosul is expected to last weeks, perhaps even longer. Isis is estimated to have as many as 5,000 fighters in Mosul, with supporters bringing the total to 7,000. The jihadists grip on the region has been weakening since January 2015, when it was at the peak of its power. Since then, it has lost almost a third of the territory it once controlled, according to security and defence analysts IHS.