The extremists were driven out of the eastern half of Mosul, the country's second-largest city, in January.
Iraqi officials gave no details of what kind of poison gas may have been used, and said the injuries were minor.
Mosul west of the Tigris river is smaller but more densely populated, including the narrow streets of the old city.
At least 13 civilians were killed by a USA -led coalition airstrike in western Mosul on Wednesday, according to a local Air Force pilot.
Federal police members ride in the back of a vehicle during clashes between Iraqi forces and Islamic State militants, in Mosul, March 25, 2017.
Lise Grande, U.N. Iraq humanitarian coordinator, says: "The level of damage in western Mosul is already far greater than in the east, even before the battle to retake the Old City begins".
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In a related development Thursday night, the Iraqi army announced the capture of Mosul's main western entrance.
ERBIL, Iraq Iraqi forces are making progress in their offensive to expel Islamic State from Mosul but face a "very complicated" urban battle as the militants hide in mosques, homes and hospitals, a USA general told Reuters on Wednesday.
- November 13: Iraq says it has recaptured Nimrud, an ancient city southeast of Mosul.
"There's still another significant number of areas within Iraq that will. need to be cleared of Daesh, and. the Iraqi security forces are getting themselves ready for that eventuality - they know that it's coming", he said. The soaring civilian death toll has also hurt morale among Iraqi troops, and raised concern that the surviving locals won't easily accept government rule if and when they ever conquer the city. "We've said for many months that as we got into the old part of the city, the dense urban terrain there, that it would be extraordinarily hard", Dorrian continued.
Six months into an Iraqi-led, US -backed military offensive to rout ISIS from Mosul, an endgame approaches.
The NGO said another airstrike struck a two-floor house in the city's western side on April 15, but there was no information about casualties.
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