Youths inspect the rubble of a damaged house April 7 after an airstrike on rebel-held Daraa al-Balad, Syria. (Alaa Al-Faqir/Reuters)

The Pentagon acknowledged in a statement Friday that it had killed more than 100 additional civilians in the nearly-three-year-old air war against the Islamic State in Iraq and Syria, bringing the admitted total deaths caused by the coalition to 484.

The new tally includes the 105 civilians killed by a U.S. airstrike in Mosul on March 17. The strike was the subject of a two-month-long investigation that resulted in the Pentagon admitting to the bombing but blaming Islamic State explosives for the majority of the casualties.

Monitoring groups, such as the U.K.-based, puts the number of civilians killed by the U.S.-led coalition in the thousands. In a tweet Friday, said the U.S.-led coalition needs more resources to track the deaths based off an apparent backlog of civilian casualty claims facing the coalition.

The statement released by the Pentagon on Friday said that from August 2014 until April 2016, it has tracked 440 civilian casualty reports in Iraq and Syria. Airwars, however, said it had tracked 1,300 in the same amount of time

While the Pentagon’s report only spans until April, Airwars has said that May appeared to be the deadliest month for civilians yet in the campaign because of U.S.-led airstrikes. The spike in deaths is likely in part because of the intense fighting in the Iraqi city of Mosul and around the Islamic State’s de-facto capital of Raqqa in Syria.

On Sunday, Secretary of Defense Jim Mattis said during an interview on “Face the Nation” that civilian casualties are “a fact of life” on the battlefields in Iraq and Syria. He added that tactics on the battlefield had shifted from “attrition” to “annihilation.” meaning that instead of pushing the Islamic State from one stronghold to the next, U.S.-backed forces are focusing on surrounding the terrorist group.

[Mosul residents say U.S.-led coalition airstrikes killed scores of people]

In Mosul, the last several neighborhoods held by the Islamic State are surrounded. Those neighborhoods — around six square miles of the city — look to hold roughly 1,000 Islamic State fighters and more than 100,000 civilians. The labyrinth of dense streets and thousands of advancing Iraqi forces puts the likelihood for civilian casualties extremely high. Last week, the United Nations warned that up to 10,000 civilians a day could flee from those areas in the coming weeks.

In Raqqa, U.S.-backed forces have nearly surrounded the city and have called for civilians to leave. It is unclear how many civilians are still in the city but Col. Ryan Dillon, a spokesman for the U.S.-led coalition, said Thursday that they are steadily leaving, and more than 100,000 displaced people are living in camps in the surrounding countryside.