The union representing a majority of workers at Canada Post has issued a 72-hour notice of job action as it tries to bargain a collective agreement with the Crown corporation.

The Canadian Union of Postal Workers said the notice spells out what actions it is planning but stops short of a full-blown walkout.

CUPW national president Mike Palecek said Canada Post forced the labour disruption by refusing to accept a request from the federal labour minister to continue negotiations with the help of a special mediator.

A spokesperson for the agency said that wasn't the case.

A strike mandate was set to expire at midnight on Thursday. That's when the federal government stepped in, saying it would appoint a special mediator in hopes of breaking the stalemate.

Labour Minister MaryAnn Mihychuk offered the mediation service late Wednesday after months of bitter disagreement, and encouraged both sides to come together.

"I expect both parties to work with this special mediator to come to a resolution and avoid a work stoppage," Mihychuk said in a brief statement.

"I continue to closely monitor the situation."

Officials from both Canada Post and the Canadian Union of Postal Workers appeared to go into a media lockdown shortly after the minister's announcement, refusing to say whether there was any room for movement in the talks.

As of late Wednesday, the union was accusing Canada Post of stalling the talks by continuing to make unacceptable demands.

The Crown agency also complained Wednesday that union bargainers were being unrealistic.

"The union continues to press for more than $1 billion in demands with no appreciation for the current and troubling future state of the postal service caused by declining mail volumes and increasing pension obligations," said Canada Post spokesman Jon Hamilton.

"Canada Post remains committed to negotiating agreements that are fair to our employees, and allow us to continue to provide affordable pricing and service to Canadians."

A pay equity issue pitting rural and urban carriers against each other and a proposed move from a defined benefit pension plan to a defined contribution plan for new employees are the main stumbling blocks in the dispute.

The union claims rural postal workers earn, on average, nearly 30 per cent less than city carriers.

"Canada Post's proposal on pay equity (for rural carriers) was nothing more than an attempt to complicate and delay that process," the union told its members in a statement.

"Canada Post wants to drag out pay equity with binding arbitration, a process that could take years or even decades."

Other issues include "precarious part-time and temporary employment, no improvements in staffing, the ability to close all 493 protected CUPW staffed retail locations eliminating up to 1,200 full-time jobs," the union said.

The Crown corporation also had the option to lock out workers after Thursday.