Vancouver and Niagara Falls are the latest cities hit by the rotating postal strikes that have been shifting across the country all week.

Thousands of unionized Canada Post workers in both cities walked off the job Friday morning, joining fellow employees in Fundy, N.B. and Sudbury, Ont. already involved in the job action.

"Mail and parcels will not be delivered or picked up in the impacted areas while the union continues their strike activity," Canada Post said in a customer update Friday. "Thank you for your continued patience."

The Canadian Union of Postal Workers filed a strike notice last week after 10 months of negotiations with Canada Post failed to reach a new collective agreement.

A key issue in talks has been how to manage the heavier workload brought on by the rise in online shopping, according to CUPW, which represents 50,000 Canada Post workers. Some employees said they have had to work up to 16-hour shifts just to finish their deliveries.

The union is also demanding wage increases and better health and safety benefits.

"Canada Post did not leave us with any option. We do not want to strike, but we do not have any choice left," striking postal employee Anju Parmar said outside a sorting facility in Richmond.

In a statement issued Wednesday, Canada Post said it has made "significant offers" to CUPW that include raises, job security and benefit improvements, and not asked for any concession in return.

The Crown corporation has said it is scrambling to work through mail and parcel backlogs leftover from previous strikes, including the two-day walkout in Toronto that ended at midnight Thursday. Tens of thousands of letters and parcels have been temporarily delayed so far.

Victoria was one of the first cities hit by the rotating strike.

There were questions about whether the job action would impact B.C.'s electoral reform referendum, though election officials said there should be plenty of time for voters to receive their ballots and return them before the Nov. 30 deadline.

With files from CTV Vancouver's Breanna Karstens-Smith and The Canadian Press