TORONTO -- Ontario's police watchdog says it laid criminal charges against 20 police officers last year, up from 16 officers in 2015.

The Special Investigations Unit, which probes incidents involving police where there has been a death, serious injury or allegations of sexual assault, detailed the figures in its annual report published Thursday.

The SIU says it closed 296 cases last year, up from 255 cases in 2015.

Seventeen of the cases closed in 2016 resulted in charges against officers, including assault, sexual assault, dangerous driving and criminal negligence causing death.

The SIU also says 38 per cent of last year's completed cases were closed by memo, which means the SIU decided quickly there was nothing to investigate under its mandate because the injury involved was not considered serious, or it wasn't caused directly or indirectly by a police officer's actions.

The SIU says it opened 327 new investigations last year, an increase of almost six per cent from 2015.

Injuries to a person in custody were the most common cause of new SIU investigations, making up 60 per cent of cases, while 13 per cent stemmed from sexual assault allegations and two per cent from firearm deaths.

The annual report was published after a sweeping review of police oversight in Ontario recommended, among other things, that the SIU report publicly on each of its cases and make public past reports on cases where police have killed someone.