An Iraqi unit with U.S. and Australian advisers has been hit by mustard gas by ISIS forces.

Six soldiers suffered breathing problems and another 19 were forced to seek treatment after ISIS militants launched the gas attack against the US-backed unit in the city of Mosul, northern Iraq.

It is the second chemical attack against the Iraqi troop in two days, after an officer with the anti-terrorism unit said ISIS fired a rocket loaded with chlorine at the al-Abar neighborhood in western Mosul.

An Iraqi unit with U.S. and Australian advisers has been hit by mustard gas by ISIS forces (a member of the Iraqi forces wears a gas mask in the old city of Mosul on April 16)

Six soldiers suffered breathing problems and another 19 were forced to seek treatment after ISIS militants launched the gas attack against the US-backed unit in the city of Mosul, northern Iraq

Gas masks and other equipment is now being distributed to forces in case of future gas attacks.

The allied Iraqi forces are battling ISIS and local officials say more than half of western Mosul has been retaken from the extremists.

ISIS was driven out of the eastern half of Mosul, Iraq's second-largest city, in January.

But hundreds of thousands of Iraqi civilians are still trapped in the ISIS-held parts of the city which could turn into the worst humanitarian 'catastrophe' in the war against the militants, the United Nations warned on Tuesday.

'If there is a siege and hundreds of thousands of people don't have water and don´t have food, they will be at enormous risk,' U.N. humanitarian coordinator for Iraq Lise Grande told Reuters in a telephone interview.

Smoke billows from Mosul's Old City on April 17, 2017, during an offensive by Iraqi security forces to recapture the city from ISIS

An Iraqi federal policeman uses a helmet on a stick to try and draw fire from an Islamic State sniper in an attempt to make him reveal his position during the battle to recapture west Mosul on April 13

'We could be facing a humanitarian catastrophe, perhaps the worst in the entire conflict,' she added.

Mosul, the largest city in northern Iraq, was captured by the ultra-hardline Sunni Muslim fighters in mid 2014.

Iraqi government forces have taken back most of it in a U.S.-backed offensive launched in October, including the half that lies east of the Tigris river.

The militants are now surrounded in the northwestern quarter including the historic Old City, using booby traps, sniper and mortar fire against the assailants.

Iraq's army has built a new pontoon bridge over the Tigris river south of Mosul, after flooding blocked all crossing points, to create an escape route for families fleeing fighting between government forces and Islamic State.

The army had dismantled makeshift bridges linking the two parts of Mosul due to heavy rain on Friday, forcing residents fleeing the city to use small boats.

Mosul's permanent bridges were largely destroyed during a six-month military campaign to seize back Mosul from the Sunni Muslim Islamists.

A member of the Iraqi forces wears a gas mask as he stands by rocket-propelled grenade launchers in the old city of Mosul on April 16

Long queues formed at the new bridge on Tuesday with families crossing in public buses, trucks and taxis.

Aid shipments also resumed to the Hammam al-Alil camp, southwest of Mosul, the main arrival point for people fleeing the fighting.

Deliveries from Erbil, located some 80 km (50 miles) east in peaceful Iraqi Kurdistan, where aid agencies are based, had stopped due to the flooding.

'Everything is back to normal,' said a spokeswoman for the U.N. refugee agency UNHCR.

Some 20,000 people have escaped from Mosul in the past four days, fewer than before due to the lack of transport, the UNHCR said in a report. Almost 330,000 people have fled Mosul since Iraq started an operation to expel Islamic State in October.

They were some of the around 400,000 people still in western Mosul where military forces are trying to dislodge the militants from the Old City.

Fighting continued in the Old City where heavy smoke could be seen from the area of the Grand al-Nuri Mosque, from where Islamic State leader Abu Bakr al-Baghdadi declared a 'caliphate' spanning parts of Iraq and Syria.

Islamic State is talking to Al-Qaeda about a potential alliance to form a combined terror group as Iraqi troops close in on jihadis in Mosul (demonstrators chant pro-ISIS slogans as they wave the group's flags in front of the provincial government headquarters in Mosul)

Aircraft, helicopter and artillery opened fire, while gunfire could also be heard at several positions of Iraq's federal police near the Old City.

'They (Islamic State militants) carry out attacks on our defensive lines, but each time we repel them and they run away, leaving bodies of their dead fighters behind,' Lieutenant Colonel Hussein Lazim Zghayer said of the force's 9th division.

'Minutes ago, they launched an attack and we responded by shelling them with mortar rounds, killing two of them and their bodies were left in front of our defensive lines,' he said earlier today.

But after months of fighting, it appears that the government forces are winning after ISIS turned in desperation to Al-Qaeda to seek a potential alliance.

Messengers representing ISIS' leader Abu Bakr al Baghdadi and his al-Qaeda counterpart Ayman al Zawahiri have discussed what is understood to be a potential merger deal for a combined terror group as the Iraqi troops close in on jihadis in Mosul.

This month it was reported the two terror groups had already formed a partnership in Libya amid mounting pressure on the jihadi movements and a leaked memo suggested they could be working together as one organisation by 2021.