TORONTO -- More than 400 employees at the Toronto Zoo have walked off the job to back their contract demands and the facility will be closed to the public as of Thursday morning.

The Canadian Union of Public Employees Local 1600 said the walkout began at midnight Wednesday at Canada's largest zoo after the two sides failed to come to terms on the key issue of job security.

"We are incredibly disappointed to have to take strike action, but the Toronto Zoo's refusal to move on job security left us with no alternative," local president Christine McKenzie said in a statement.

But a spokeswoman for the zoo said it has been "more than fair and reasonable."

Jennifer Tracey said in a statement that the union "has not provided any flexibility" and the zoo's "fair and reasonable" proposals include a wage increase and "satisfactorily address the issues of job security."

Tracey added that the facility is a not-for-profit charitable organization and the union demands are "simply unaffordable and do not reflect the financial realities facing the zoo."

She said the employees are "well paid, receive an employer-paid pension plan, multiple weeks of vacation, and are covered by excellent health and dental benefits which includes more than 100 sick days per year."

The union said picket lines would go up at the zoo's entrances in east end Toronto and that workers were being asked to show up at their regular shift times, but to report for picket duty.

McKenzie said the union's bargaining committee remains ready to quickly resume negotiations and urged zoo management to provide their negotiating team with the "flexibility they need to conclude bargaining."

"We have numerous animals on the verge of giving birth, a new health centre supposed to open, and thousands of animals that won't be getting the level of care they should be getting," she said.

"Ultimately, that ball is in the employer's court."

Tracey said the zoo's animals will be taken care by "highly qualified management staff, many who were former keepers themselves."

The 400 workers include zookeepers, maintenance, administration, ride operators, public relations staff and concession workers.