TORONTO - Ontario ombudsman Andre Marin is launching an investigation into the use of excessive force by prison guards against inmates in the province's jails.

More than 100 complaints have come into his office from inmates who say correctional officers assaulted them and that may be just the "tip of the iceberg," the government watchdog said Tuesday.

What's even more disturbing are the allegations that the violence has been covered up or ignored by a "code of silence" within the prison, he said.

"We're talking about cases that are quite serious in fact," Marin said in an interview.

"For example, an inmate alleges that a correctional officer choked him until he was unconscious, and then subsequently there was a failure to file a use of force report by the guard."

Another inmate alleged that he lost three teeth after being punched by a guard, but no documents were filed about the incident and no charges pursued against the corrections officer.

The probe will look at the government response to inmate complaints, including how it conducts investigations and enforces policies on the use of force, he said.

So far, he's "unimpressed" with the lack of urgency by the government in dealing with these incidents, which are supposed to be investigated by the Correctional Investigation Security Unit.

"They're not being reported, therefore they're not being investigated and I'm finding that very troubling," Marin said.

Ministry officials weren't surprised by the allegations, he added. In fact, they said they knew there was a problem.

Correctional Services Minister Jim Bradley wasn't available for comment, but his spokesman Joe Kim said in an email that the ministry will co-operate fully with Marin's investigation.

Inmates aren't the "most savoury types of characters," but once they're incarcerated, they're at the mercy of the state, Marin said. Such violent incidents may also affect their chances at rehabilitation.

"These are vulnerable people under public care and custody, and we need to ensure that they're not being abused," he said. "And if they are, that the provincial government is taking it seriously and investigating it."

Marin said he's assigning his special ombudsman response team to conduct the investigation, but isn't sticking to the usual 90-day deadline.

"Our suspicion is that right now, we've only hit the tip of the iceberg," he said.

"And I don't want to confine myself to a deadline right now, because my concern is that we're going to be hearing a lot more about these cases come up as we announce the investigation."