US Central Command (CENTCOM) said in a statement on Friday that it added an additional 132 civilians deaths to its April report, a sharp increase from 352 it previously reported in late April.
However, that total, which only includes civilian deaths through April, was still far short of what non-governmental organisations and monitors have estimated.
Airwars, a London-based collective of journalists and researchers that tracks civilian deaths in Syria and Iraq, estimated more than 3,800 non-combatants have been killed since the US-led coalition's operations began in August 2014.
CENTCOM's estimate includes 105 civilians killed in a US-led air raid in March against a building in the Iraqi city of Mosul, the single deadliest incident for civilians arising from a coalition attack since anti-ISIL operations began in Iraq and Syria nearly three years ago.
Separately, Al Jazeera's sources recently said more than 120 civilians were killed in less than a week as Iraqi forces - backed by coalition air power - move to take the remaining pockets of territory held by the Islamic State of Iraq and the Levant (ISIL, known as ISIS) group in Mosul.
In a statement emailed to Al Jazeera earlier on Friday by the Operation Inherent Resolve press office, the coalition said it "is aware of allegations of civilian casualties".
It added that the "coalition and Iraqi security forces are making every attempt to safeguard civilians as they liberate the city from ISIS terrorists who are using snipers to target civilians trying to flee the city … The coalition takes all allegations of civilian casualties serious and will assess the allegations".
The battle to recapture the last stronghold of ISIL in Iraq has now entered its eighth month.
Iraqi government forces, backed by US advisers, artillery and air support, have cleared the east and most of western Mosul and are now focused on controlling the Old City with Iraqi civilians paying a heavy price.
"We moved out and got frightened by heavy air strikes," one civilian who escaped the fighting in western Mosul told Al Jazeera. "We fled after our house was destroyed by mortar shelling."
Nearly 200,000 civilians are caught in an area of about eight square-kilometres.
Al Jazeera's Osama bin Javaid, reporting from Erbil, just east of Mosul, said observers are pushing the Iraqi military and the US-led coalition to take care of civilians, despite the intensity of combat against ISIL.
"Saving people is proving to be easier said than done," Javaid said. "Aid workers and rights groups have been repeating their concerns that in the process to push ISIL out, Iraqi forces must make sure that civilians are not caught in the crossfire."
Meanwhile in Syria, the UK-based Syrian Observatory for Human Rights estimates that in the month from April 23 to May 23, 225 civilians, including dozens of children, were killed, the heaviest monthly toll since 2014.
Last week, the Observatory said at least 13 people were killed in suspected US-led coalition air raids on the ISIL-held city of Raqqa and suspected rocket attacks by a Kurdish group fighting ISIL.
The SDF, which includes the powerful Kurdish YPG armed group, said in May it plans to launch the final assault on Raqqa in early summer.
It has been encircling the city, ISIL's de facto capital in Syria, since November.
The UN said in a report that on May 14, at least 23 farm workers, including 17 women, were reportedly killed when air raids hit al-Akershi village in a rural area of eastern Raqqa province.
Other air raids on two residential areas of the ISIL-controlled city of Abo Kamal in eastern Deir Az Zor province the following day, May 15, reportedly killed at least 59 civilians, including 16 children and 12 women, and wounded 70 others.
UN human rights chief Zeid Ra'ad al-Hussein said in a statement last week that the "rising toll of civilian deaths and injuries already caused by air strikes in Deir Az Zor and Raqqa suggests that insufficient precautions may have been taken in the attacks,"
He added that "just because ISIL holds an area does not mean less care can be taken. Civilians should always be protected, whether they are in areas controlled by ISIL or by any other party."
Source: Al Jazeera and news agencies