The union representing Ontario's correctional officers is urging them to press for criminal charges against inmates who commit even minor assaults on guards.

Monte Vieselmeyer of the Ontario Public Service Employees Union says the number of assaults on staff has been going up, particularly in the London area, where an inmate allegedly punched a correctional officer in the face last week.

Vieselmeyer says offenders who assault officers are normally put in segregation.

However, he says that's no longer feasible in overcrowded Ontario prisons.

He says reporting even minor incidents to police is a way to penalize offenders.

It is also a way to raise awareness of institutional challenges.