The US-led coalition against the Islamic State (IS) group says it has unintentionally killed more than 1,300 civilians in Iraq and Syria since 2014.
A UK-based monitoring group says the true toll is much higher, estimating up to nearly 13,000 civilian fatalities.
The US-led action began after IS took over huge areas of territory.
It imposed brutal rule over millions of people who fell under its control and has carried out or inspired deadly attacks around the world.
The latest figure provided by the coalition is slightly higher than its previous admission eight months ago of 1,100 civilian deaths. It says it is still assessing 111 more possible cases of civilian fatalities.
The latest acknowledgement stands in stark contrast to the claims of human rights and monitoring groups, which say the actual death toll is many times higher.
Amnesty International's senior crisis response advisor Donatella Rovera accused the US-led coalition of remaining "deeply in denial" about the true scale.
"Today's acknowledgement of further civilian deaths underscores the urgent need for thorough, independent investigations that can uncover the true scale of civilian casualties caused by coalition strikes, examine whether each attack complied with international humanitarian law and provide full reparation to victims," she said.
Last month, an investigation by activists concluded that more than 1,600 civilians were killed in coalition attacks on the Syrian city of Raqqa alone during a five-month campaign to oust IS in 2017.
Raqqa had been the de facto capital of the jihadists' self-proclaimed "caliphate".
At that time, a coalition spokesperson told the BBC that "any unintentional loss of life during the defeat of [IS] is tragic. However it must be balanced against the risk of enabling [IS] to continue terrorist activities, causing pain and suffering to anyone they choose".
The coalition "methodically employs significant measures to minimise civilian casualties", the spokesperson said, and "always balances the risk of conducting a strike against the cost of not striking".
The monitoring group Airwars, which tracks allegations of civilian deaths, says the coalition may have actually killed between about 8,000 and 13,000 civilians to date.