(CNN) -- Following weeks of negotiations, 3,800 Air Canada customer service and sales employees went on strike early Tuesday morning.
The sticking point was pensions, said Jerry Dias of the Canadian Auto Workers, the strikers' union.
Air Canada "refused to remove major concessionary demands on the existing pension plan," the union said in a statement.
The union had previously set a deadline of 11:59 p.m. on Monday to reach an agreement.
Air Canada emphasized that previously scheduled flights would continue as planned.
"Despite the strike, Air Canada is operating its full schedule," Air Canada Chief Operating Officer Duncan Dee said in a video the company posted on YouTube.
"We have a contingency plan that will have 1,700 staff at airports and call centers to assist our customers," he said.
Nonetheless, rather than rely on call centers for information, Dee advised customers to check the company's website.
"This strike is unfortunate -- not only for our members, but also for Air Canada passengers who may be inconvenienced," union President Ken Lewenza said in a statement.