However, U.S. -led coalition forces have made major gains against the group, both in Mosul and in the Syrian city of Raqqa, the last major IS stronghold in that country.
"Our units are continuing to advance. and entered Al-Saha al-Oula and Al-Zinjili and Al-Shifaa neighbourhoods and the Republican Hospital", said Joint Operations Command spokesman Yahya Rasool.
The investigation comes amid broader claims that US forces under President Donald Trump are killing more civilians as the military fulfills a plan to "annihilate" the Islamic State group.
The Iraqi commander also added that "the enemy has lost many of its defense capabilities" these past months.
A number of other houses in the area were also destroyed by clashes between IS fighters and USA -backed Iraqi forces around the same time, according to residents interviewed by The Associated Press, but the Pentagon investigation looked into the single March 17 airstrike that hit at 8:24 a.m. local time. "The fighting is extremely intense ... the presence of civilians means we have to be very cautious", he said, speaking on condition of anonymity.
Iraqi forces have launched a broad assault on parts of Mosul, closing in on the remaining pockets of territory held by ISIL, amid growing concerns over the safety of hundreds of thousands of civilians at risk of being caught in the crossfire.
On Saturday, U.S. -backed Iraqi forces began a new offensive to recapture the Old City from three directions.
Unlike past urban battles against IS, in Mosul Iraq's government asked civilians to remain in their homes in order to avoid massive numbers of displaced families requiring camps and other assistance.
She said that the United Nations estimates there are between 180,000 and 200,000 civilians in jihadist-held areas of Mosul, the majority of them in the Old City area. The district is home to much of Mosul's ancient heritage, including the Iconic leaning minaret of the al-Nuri Mosque where IS leader Abu Bakr al-Baghdadi declared an Islamic caliphate stretching across IS-held lands in Syria and Iraq in June 2014. "In a single day they buried 30 bodies", she said.
The push inside the Old City coincides with the start of the holy fasting month of Ramadan.
Since Friday, the government has been working to get civilians out of the areas in plans to attack, dropping leaflets to alert citizens to "safe passages" where they could flee with the help of "guides, protectors and (transportation)".
Reports say almost 760,000 people have already escaped the embattled Iraqi city over the past months.
The IRGC is the main backer of the Iraqi Shi'ite paramilitary force known as Popular Mobilisation.
The insurgents are also retreating in Syria, mainly in the face of US-backed Kurdish-led forces.
On March 17 alone, the coalition dropped 81 bombs in western Mosul, including 12 on al-Jadida neighborhood, an area just about two square kilometers in size, according to Scrocca.