OTTAWA -- Canada has added its voice to calls for the international community to do more as aid groups brace for a potential humanitarian crisis near the Iraqi city of Mosul.

Iraqi and Kurdish forces this week launched a massive assault on Iraq's second-largest city to free it from the Islamic State of Iraq and the Levant. The operation had been expected for months and is seen as a pivotal moment in the fight to defeat ISIL.

In anticipation of the attack, the United Nations as well as a number of aid groups have been pre-positioning supplies for months to deal with an expected flood of civilians fleeing the fighting. Mosul is home to an estimated 1.2 million people.

But those efforts have been marred by a shortage of funding. The UN asked the international community in July for $284 million in aid for the Mosul offensive, but less than half has been pledged.

Canada is the fifth-largest donor of humanitarian assistance to Iraq, after the United States, United Kingdom, Germany and European Commission. That includes $150 million in humanitarian assistance promised in July.

International Development Minister Marie-Claude Bibeau said the money, to be spread over three years, is going to groups like the International Committee of the Red Cross and Oxfam to use as needed.

While not earmarked specifically for Mosul, she said some has already been used to support preparations for the Mosul attack.

That includes providing emergency food rations through the World Food Programme and ICRC and helping the International Organization for Migration with camps and emergency shelter.

But while touting Canada's contribution, Bibeau said much more is needed from the international community to prevent a tragedy.

"Civilians under siege are likely to be at the highest risk of hunger, lack of medical assistance and human rights violations," she said. "This is a global responsibility and all donors must step up to respond to what is now unfolding in Mosul."

The request for funding and advanced preparations were a direct result of the catastrophe in Fallujah earlier this year.

Humanitarians groups were overwhelmed as tens of thousands of people fled the city into the surrounding countryside to escape fighting between Iraqi and ISIL forces.

Aid workers on the ground have not yet seen a rush of civilians from Mosul since the operation to free the city only started on Monday. They say the fighting has so far been focused on largely abandoned villages around the city.

"For us, we expect the big influx when the fighting gets to the city," Oxfam's country director in Iraq, Andres Gonzalez Rodriguez, said in an interview from Erbil. "The worst is yet to come."

The UN says it is ready to accommodate up to 60,000 in 27 camps and emergency sites scattered around Mosul, but the fear is that hundreds of thousands could flee all at once.