The advance of Iraqi government troops slowed on Sunday in the last push to drive ISIS militants from remaining pockets of Mosul, Iraqi military officers said.

On Saturday, U.S.-backed Iraqi forces began a new offensive to recapture the Old City from three directions.

Hours after announcing the push, the government said two military officers were killed in clashes in the Shafaa neighbourhood on the Tigris River.

ISIS militants have deployed snipers, suicide car bombers and suicide attackers on foot, the officers said.

The officers described the advance on Mosul's Old City as "cautious" and the clashes on Sunday as "sporadic" without giving details on casualty figures from either side.

The ISIS hold on Mosul has shrunk to just a handful of neighbourhoods in and around the Old City district where narrow streets and a dense civilian population are expected to complicate the fight there.

The United Nations humanitarian co-ordinator in Iraq said civilians are being hit hardest as the military assault against ISIS militants in Mosul enters its final phase.

Lise Grande condemned ISIS for using civilians to shield themselves from attack. ​She said families who are already facing severe shortages of water and electricity are now in extreme danger.

"We know that [ISIS] is directly targeting families as they try to escape, we know that there are very limited stocks of food and medicines, we know that there are severe shortages of water and electricity," she told BBC.

"All of the evidence points to the fact that the civilians who are trapped in these neighbourhoods and districts are in grave danger."

"There have been more than 12,000 people who have been referred by the United Nations and frontline health partners to hospitals and trauma stabilization points. Many of these people have been shot. Many of them have been shot in the back," Grande said.

Mosul's eastern half was declared liberated in January and the push for the city's west began the following month.