Iraqi forces advance as they battle with Islamic State militants, in western Mosul, Iraq March 6, 2017. REUTERS/Suhaib Salem

By Isabel Coles and Maher Chmaytelli

MOSUL/BAGHDAD, Iraq (Reuters) - U.S.-backed Iraqi forces captured the second of Mosul's five bridges on Monday, giving a boost to their onslaught on Islamic State's remaining stronghold in the western part of the city.

All of Mosul's five bridges over the Tigris have been destroyed but their capture facilitates the movement of forces progressing alongside the river,which cuts Mosul in two.

The bridge seized, al-Hurriya, is the second after one located further south. Its capture shields the back of the forces advancing towards a nearby government buildings complex.

"We control the western end of the bridge," said a senior media officer with Rapid Response, the elite unit of the Interior Ministry leading the charge towards the complex.

Recapturing the site would help Iraqi forces attack the militants in the old city. It would also mark a symbolic step towards restoring state authority over Mosul, even though the buildings are destroyed and not being used by Islamic State.

The battle of Mosul, which started on Oct. 17, will enter an more complicated phase in the densely populated old city.

Civilians have been displaced in greater numbers in the past days, as the fighting rages in the middle of residential neighbourhoods where populations have already been suffering for months from food, water and electricity shortages.

Iraqi forces captured the eastern side of Mosul in January after 100 days of fighting and launched their attack on the districts that lie west of the Tigris on Feb. 19.

Defeating Islamic State in Mosul would crush the Iraqi wing of the caliphate declared by the group's leader, Abu Bakr al-Baghdadi, in 2014, over parts of Iraq and Syria.

The Iraqi foreign ministry meanwhile expressed "deep relief" at U.S. President Donald Trump's decision to remove Iraq from a list of countries targeted in a U.S. travel ban.

A U.S.-led coalition is providing key air and ground support to the Iraqi forces in the battle of Mosul.

"The decision is an important step in the right direction, it consolidates the strategic alliance between Baghdad and Washington in many fields, and at their forefront war on terrorism," the ministry said in a statement.

Trump is expected to sign a new executive order on Monday banning travel to the United States by citizens of six Muslim-majority nations after his controversial first attempt was blocked in the courts, a White House source said

Baghdadi, Islamic State's leader, proclaimed the caliphate from Mosul's grand Nuri mosque in the old city center which is still under his followers' control.

"In the coming hours our forces will raise the Iraqi flag over the governorate building," Federal Police Brigadier General Shaalan Ali Saleh told Reuters.

The militants have barricaded streets with civilian vehicles and rigged them with explosives to hinder the advance of Iraqi forces were also met with sniper, machinegun and mortar fire, as well as explosives dropped from light drones.

Federal Police units who also taking part in the offensive are using similar drones to hit the militants.

The Iraqi military believes several thousand militants, including many who travelled from Western and central Asian countries, are hunkered down among the remaining civilian population, which aid agencies estimated to number 750,000 in western Mosul at the start of the latest offensive.

The militants are using suicide car bombers, snipers and booby traps to counter the offensive waged by the 100,000-strong force of Iraqi troops, Kurdish peshmerga fighters and Iranian-trained Shi'ite Muslim paramilitary groups.

They were also reported to have fired rockets and mortar rounds filled with toxic agents from the western side of the city to the eastern, government-controlled side.

More than 40,000 fled their homes in the past week, bringing the total number of those of displaced since the start of the offensive to nearly 210,000, according to the United Nations.

Aid agencies have expressed concern that camps to accommodate people fleeing are nearly full.

The United Nations last month warned that more than 400,00 people, more than half the remaining population in western Mosul, could be displaced.

(Writing by Maher Chmaytelli; Editing by Angus MacSwan)