US and allied strikes killed between 3,923 and 6,102 non-combatants in both countries last year, Airwars, a UK-based group tracking allegations of civilian casualties said on Thursday, calling the 2017 campaign the "deadliest year yet for ordinary Syrians and Iraqis".
The group said its estimates were based on publicly available data.
The sharp rise last year was because of intense fighting in densely populated urban areas, as Iraqi troops and Syrian Kurdish-dominated forces, both backed by the US-led coalition, fought respectively to remove the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group from Iraq's Mosul and Syria's Raqqa.
The administration of US President Donald Trump, in power since January 20, 2017, was also partly to blame, Airwars said.
"This unprecedented death toll coincided with the start of the Trump presidency, and suggested in part that policies aimed at protecting civilians had been scaled back under the new administration," the monitor said.
In Syria, the toll for civilians killed in coalition air raids quadrupled compared with 2016, with between 2,786 and 4,374 deaths likely killed in 2017, the group said.
In Iraq, deaths were up by 87 percent, with at least 1,128 non-combatants dying in more than 100 air attacks, it added.
While US officials call the campaign "the most precise in history", according to Airwars, "the urban battlefields laid waste by bombs, artillery and improvised explosives told another story".
Coalition forces carried out some 11,573 air and artillery attacks in 2017 and dropped a total of 39,577 bombs and missiles against ISIL.
More than 70 percent of those raids were in Syria, largely because of the campaign to capture Raqqa.
By the time the city was seized from ISIL, more than 1,450 civilians were killed, Airwars said, and 80 percent of the city was left uninhabitable.
Kinda Haddad, the head of Airwars' Syria team, said the coalition repeated Russian and Syrian government "tactics of 'siege, bomb and evacuate' in order to achieve the stated aims of defeating" ISIL.
Coalition raids, in the nine-month battle for Mosul, which began in October 2016, claimed between 1,066 and 1,579 lives, Airwars said.
That was "the biggest urban assault since World War Two", it said, and left the city reduced to rubble.
An Airwars researcher said that by the end of the campaign in June, "it sometimes seemed that the coalition were attacking everything in order to eradicate ISIS from the city".
The coalition said only 93 of its raids caused deaths and injuries, but Airwars said it had identified 673 other such incidents.
In addition to the deaths, some 2,433 civilians were wounded in the attacks.
The group said coalition-linked civilian casualties in 2017 "far" outnumbered those attributed to Russia.
However, it said it had to suspend its assessments of Russia from March last year because of the "massive rise" in coalition actions and limited resources.