The Pentagon said on Sunday at least 352 civilians were killed as a result of US-led campaign against the Islamic State (IS) in Iraq and Syria from August 2014 to March 2017.
In its monthly report of assessment of civilian casualties, the Pentagon said it was still assessing 42 reports of civilian deaths, Xinhua news agency reported.
According to the Pentagon, 45 civilians were killed between November 2016 and March 2017.
In addition, the US military reported 80 civilian deaths from August 2014 to the present which were not previously announced.
"Although the coalition takes extraordinary efforts to strike military targets in a manner that minimises the risk of civilian casualties, in some incidents casualties are unavoidable," said the Pentagon.
The Pentagon's figures contradict the assessment by London-based Amnesty International, which estimated that about 300 civilians have been killed in 11 coalition air strikes in Syria alone.