BAGHDAD (Reuters) - Prime Minister Haider al-Abadi said on Tuesday it would take three months to remove Islamic State from Iraq, as U.S.-backed forces battle to dislodge the militants from Mosul, their last major stronghold in the country.

Abadi had previously pledged the northern city would be retaken by the end of this year. But the operation has been slowed by concern to avoid casualties among civilians, who have mostly stayed in their homes rather than fleeing as was initially expected.

Asked to respond to comments by the commander of a U.S.-led coalition supporting Iraqi forces that it would take as long as two years to eliminate Islamic State and its cells in Iraq and neighboring Syria, Abadi said:

“The Americans were very pessimistic. They used to talk about a really long period, but the remarkable successes achieved by our brave and heroic fighters reduced that. I foresee that in Iraq it will take three months.”

Overnight the coalition bombed the last remaining bridge connecting the eastern and western parts of Mosul “to reduce enemy freedom of movement”, a spokesman said on Tuesday.

A statement published by Amaq, a news agency supporting Islamic State, said the bridge was now completely out of service, and an unconfirmed video circulated online showed a segment of the span had fallen into the river.

The United Nations has previously expressed concern that the destruction of Mosul’s bridges could obstruct the evacuation of civilians. Up to 1.5 million are thought to remain inside.

More than two months into the Mosul operation, elite Iraqi soldiers have retaken a quarter of the city, but entered a planned “operational refit” this month.

A U.S. battlefield commander told Reuters on Monday that Iraqi forces would resume their offensive in the coming days, in a new phase of the operation that will see American troops deployed closer to the front line inside the city.

Mosul, the largest city seized by Islamic State anywhere across the once vast territory it controlled in Iraq and neighboring Syria, has been held by the group since its fighters drove the U.S.-trained army out in June 2014.

Besides Mosul, Islamic State still controls the towns of Tel Afar and Qaim as well as Hawija and the surrounding area.

The city’s fall would probably end Islamic State’s ambition to create a self-styled caliphate, but the fighters could still mount a more traditional insurgency in Iraq, and plot or inspire attacks on the West.