Americans with severe mental illness are 16 times more likely to be killed by police than other civilians, a study by an advocacy group said on Thursday.

Official and unofficial accounts of the hundreds of Americans killed yearly in encounters with police show that at least a quarter of those slain are severely mentally ill, the report by the nonprofit Treatment Advocacy Center said.

"At this rate, the risk of being killed during a police incident is 16 times greater for individuals with untreated mental illness than for other civilians approached or stopped by officers," it said.

The study estimated that just under 4 percent of American adults were severely mentally ill but generated 10 percent of calls for police service. They occupy at least one in five prison and jail beds, it said.

The study comes during a U.S. debate on the use of deadly force by police, fueled by the deaths of unarmed men in New York, Missouri, South Carolina and elsewhere.

The Treatment Advocacy Center based its estimate on data gathered by media organizations, nonprofits and bloggers. It said the data pointed to an annual average of about 1,000 people a year killed in officer-related shootings.

A Washington Post database reports that 917 people have been fatally shot by police this year.

There are no reliable U.S. government numbers on officer-related killings. In response to criticism over the lack of information, the FBI is setting up a database on police killings.