US expects IS militants to use chemical weapons in fight for Mosul

The United States expects the Islamic State group to use crude chemical weapons as it tries to repel an Iraqi-led offensive on the city of Mosul, US officials say, although adding that the group's technical ability to develop such weapons is highly limited.

US forces have begun to regularly collect shell fragments to test for possible chemical agents, given the use of mustard agent by IS militants in the months before Monday's launch of the Mosul offensive, one official said.

In a previously undisclosed incident, US forces confirmed the presence of a sulphur mustard agent on IS munition fragments on 5 October, a second official said.

IS militants had also targeted local forces, not US or coalition troops.

"Given ISIL's reprehensible behavior and flagrant disregard for international standards and norms, this event is not surprising," the second official told Reuters, speaking on condition of anonymity, and using an acronym for the IS group.

American officials do not believe that IS has been successful so far at developing chemical weapons with particularly lethal effects, meaning that conventional weapons are still the most dangerous threat for advancing Iraqi and Kurdish forces - and any foreign advisers who get close enough.

Sulphur mustard agents can cause blistering on exposed skin and lungs. At low doses, however, that would not be deadly.

Roughly 5,000 US forces are in Iraq. More than 100 of them are embedded with Iraqi and Kurdish Peshmerga forces involved with the Mosul offensive, advising commanders and helping them ensure coalition air power hits the right targets, officials said. Still, those forces are not at the front lines, they added.

The fall of Mosul would signal the defeat of the ultra-hardline Sunni group in Iraq but could also lead to land grabs and sectarian bloodletting between groups that fought one another after the 2003 overthrow of Saddam Hussein.

Obama warning on Mosul civilians

US President Barack Obama estimated on Thursday that perhaps one million civilians were still in Mosul, creating a challenge for Iraq and its Western backers trying to expel the group through force.

"If we aren't successful in helping ordinary people as they're fleeing from ISIL, then that makes us vulnerable to seeing ISIL return," Obama told reporters in Washington.

The International Organisation for Migration's (IMO) Iraq chief, Thomas Weiss, said on Tuesday he expected IS militants to use Mosul residents as human shields and lent his voice to concerns about the dangers of chemical agents.

The IOM had not managed to procure many gas masks yet, despite those risks, Weiss said from Baghdad.

"We also fear, and there has been some evidence that ISIL might be using chemical weapons. Children, the elderly, disabled, will be particularly vulnerable," Weiss said.

Attacking Iraqi forces are still 20 to 50 km from the city itself and US officials believe that IS is most likely to use chemical weapons later in the campaign, in what could be a difficult, protracted battle.

Baghdadi 'still in Mosul'

The leader of IS was reported to be among thousands of hardline militants still in the city, suggesting the group would go to great lengths to repel the coalition.

American officials believe some of the Islamic State groups best fighters are in Mosul.

Mosul which is Iraq's second biggest city has been under the control of IS since June 2014 after Iraqi army operatives deserted their positions in the city. 

The city is viewed as being the Islamic State group's de-facto capital in Iraq with key supply routes linking to its territory in Syria where it controls large swath of territory. 

After seizing control of Mosul in 2014 the groups leader Abu Bakr al-Baghdadi called on the worlds Muslim community to "obey" him in a show of power, after seizing control of the city for more than three weeks. 

The leader of the group militant group was reported to have fled its de-facto capital in Syria out of fear for his life.