BAGHDAD, Iraq, July 18 — An average of more than 100 civilians per day were killed in Iraq last month, the highest monthly tally of violent deaths since the fall of Baghdad, the United Nations reported today.

The death toll, drawn from Iraqi government agencies, was the most precise measurement of civilian deaths provided by any government organization since the invasion and represented a dramatic increase over daily media reports.

United Nations officials also said that the number of violent deaths had been steadily increasing since at least last summer. In the first six months of this year, the civilian death toll jumped more than 77 percent, from 1,778 in January to 3,149 in June, the organization said.

This sharp upward trend reflected the dire security situation in Iraq as sectarian violence has worsened and Iraqi and American government forces have been powerless to stop it.