The battle for Mosul has now entered its 15th week. The Iraqi Security Forces are still consolidating their gains made on the eastern bank of the River Tigris that divides the city, now the Islamic State of Iraq and the Levant (ISIL) group's last major urban stronghold in Iraq.

Pockets of resistance still exist in areas closest to the northeastern bank, such as in Rashidiya and Shraikhan, but Iraqi forces are now certainly looking to begin pushing ISIL out of west Mosul. But what will be the cost?

A bloody quagmire

In many ways, the urban fighting for Iraq's second largest city resembles an operation that a different Iraqi army undertook almost four decades ago.

In the opening days of the Iran-Iraq war, the Iraqi army was ordered to take the city of Muhammara, known as Khorramshahr to the Iranians.

The operation, known as the battle of Muhammara in Iraq, was eventually successful, but it had such bloody street-to-street fighting that it decimated the Iraqi special forces, taking them years to recover.

Though Muhammara - a smaller city than Mosul - took the Iraqi army only a month and a half to capture, the similarities between that battle and the ongoing operation in Mosul are stark.

In the battle for Mosul, Iraq has been heavily reliant on its most elite unit, the Counter Terrorism Service (CTS), which has been trained and equipped by the United States. CTS units spearheaded the operation and were the first to breach the boundaries of east Mosul.

However Iraqi forces breach western Mosul, it will largely be an infantry fight.

Recent reports and interviews with senior US military commanders, however, reveal that the CTS have been badly mauled over the past three months, with its veteran battalions "suffering upwards of 50 percent casualties".

Similarly, authorities in Baghdad have refused to release casualty figures, and even lashed out at the United Nations for releasing figures that seemed to confirm ISIL's own estimates for the first month of combat. According to ISIL, the Iraqi military, which has amassed 100,000 men in this US-backed operation, could have suffered losses close to 6,500 men by now.

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There is, of course, no way to fully verify ISIL's claims that are probably exaggerated for propaganda purposes. However, and in conjunction with the statements of US commanders as well as the UN's own figures, it is for certain that the Iraqi Security Forces, or ISF, have suffered devastating losses against a well-entrenched, determined and wily adversary.

As such, it is perhaps understandable that the Iraqi Prime Minister Haider al-Abadi failed to realise his promise of recapturing Mosul from ISIL by the end of 2016, and has now stated that this may not happen until the spring of this year.

Iraq's military options

This then leads us to the question of how Iraq intends to capture western Mosul, believed to be even more heavily defended than the eastern side, which is also still not completely under Iraqi control. Reaching the Tigris should not be seen as a "halfway mark", as war is rarely a linear affair, and retaking the western half could be even harder than the eastern bank.

To prevent ISIL reinforcements from moving between the western and eastern banks of the Tigris, US warplanes bombed Mosul's five bridges, taking them all out of action. This means that ISF sappers would have to build temporary bridges to facilitate the movement of men and materiel across the river.

To do this, they would have to contend with ISIL artillery fire, snipers and assaults against any troops attempting to land on the eastern bank of the river.

Any bridging attempts would thus involve pushing as many men across to overwhelm ISIL's ability to counter, which would be extremely costly in lives.

The ISF could also secure bridgeheads by sending marines across by boat, or dropping paratroopers in by air to disrupt ISIL defences as the troops cross, but both these methods would also be costly.

A further option would be to squeeze ISIL, also known as ISIS, between the city's western edges and the river. The ISF could redeploy to areas around Tal Afar, about 60km west of Mosul, and press on to the city from there. This would force ISIL to contemplate an assault coming from across the Tigris, as well as a land assault from the west.

This, however, would involve close coordination with the Popular Mobilization Forces, a predominantly Shia paramilitary force accused by groups such as Amnesty International of perpetrating war crimes against the Sunni Arab population. The political implications of their use in Sunni Arab-dominated Mosul could be dire, especially considering their documented human rights abuses.

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However Iraqi forces breach western Mosul, it will largely be an infantry fight. The old city of Mosul is filled with narrow streets that prevent extensive use of armour and limit their manoeuvrability, which will probably result in heavy casualties by the time operations conclude sometime in the spring.

ISIL has been preparing for this fight for years, and has turned Mosul into a death-trap. Squeezing it out of Mosul and ending its barbarous self-declared caliphate will not come cheaply, and will perhaps be the most defining battle in Iraq following the US-led invasion in 2003.

Tallha Abdulrazaq is a researcher at the University of Exeter's Strategy and Security Institute and winner of the 2015 Al Jazeera Young Researcher Award.

The views expressed in this article are the author's own and do not necessarily reflect Al Jazeera's editorial policies.