OTTAWA -- Canada Post is facing a days-long backlog of parcel deliveries as scattered walkouts by postal workers escalate across the country, the Crown corporation warned Wednesday, hours before the Canadian Union of Postal Workers was set to call a national overtime ban.

Dozens of trailers filled with parcels and packages were awaiting processing at the agency's three biggest hubs as Canada Post employees from several Quebec communities joined countrywide rotating strikes a day after about 6,000 workers walked off the job in Montreal.

"At this point there is a backlog of over 150 trailers in Toronto, Vancouver and Montreal of items waiting to be unloaded and processed, with more arriving every day," said Canada Post spokesman Jon Hamilton.

"Once processed, these items have to be delivered without overburdening our delivery employees. As a result, customers could see delays of several days."

Combined, the three key locations can process a million parcels a day from across the country, Hamilton said.

To further press its contract demands, CUPW would institute a ban on overtime for both of its major bargaining units, effective 12:10 a.m. Thursday, the union told The Canadian Press.

The ban means postal workers would refuse to work overtime hours beyond their normal eight-hour days.

"We've had it," said CUPW national president Mike Palecek. "Overburdening, overtime and overwork are all major issues in this round of bargaining."

CUPW said earlier Wednesday walkouts had started in Saint-Jerome, Vaudreuil-Dorion, Sorel, Sainte-Therese de Blainville and Valleyfield in Quebec.

Workers in Joliette, Que., had been on strike since 1 a.m. EDT, and the Prince Edward Island communities of Summerside and Charlottetown were hit by strikes that started at midnight local time.

The Montreal walkout ended Tuesday night, but another 16 communities across the country were taking part in the 24-hour strikes, the union said.

In Ontario, walkouts began in Arnprior-Renfrew and Ottawa Wednesday morning, but strikes in Fort Frances, Deep River and other communities in the province ended.

Postal workers also walked off the job at four locations on Vancouver Island.

The union and the postal service have been unable to reach new collective agreements for two bargaining units after 10 months of negotiations.

Canada Post has said it provided "significant" offers to its employees, including wage hikes.

"But they don't address a single one of our major issues, Palecek said Wednesday in a statement on the union's website.

Those issues include health and safety concerns, Palecek said, adding that the wage offers from Canada Post fall far below expected cost-of-living increases.

Last Tuesday, Labour Minister Patty Hajdu appointed Morton Mitchnick, a former chairman of the Ontario Labour Relations Board, to help the two parties resolve their differences.