The Professional Association of Foreign Service Officers escalated the rotating job action after an attempt to seek binding arbitration with the federal government broke down last week.
Canada's tourism association blamed the diplomats for walking away from binding arbitration.
"We are disappointed the union has walked away from the mediation process, just as we were profoundly disappointed that they purposefully chose this time of year for their action — when we have the highest volumes of both visitor and student visas," David Goldstein, president of the Tourism Industry Association of Canada, said in an emailed statement.
But union president Tim Edwards said it did no such thing; he said the government imposed six untenable preconditions that would have prejudiced the outcome in its favour.
"That's patently false. We're the ones who offered binding arbitration in the first place. We would have preferred that it happened without preconditions," Edwards said in an interview.
Edwards said the union agreed to three of the government's preconditions but had it accepted the other three, it would have stacked the deck in the government's favour and predetermined the outcome.
"They're the ones who walked away from the table," said Edwards. "I can't emphasize this enough. Binding arbitration is still on the table, even with the three preconditions we've agreed to."
Edwards agreed that the effect of the continuing job action, now almost two months old, is having a "mounting and severe" impact on the tourism sector as well as foreign students and seasonal workers.
The tourism industry estimates that the job action will cost it $280 million this year.
In recent weeks, it has shown little sympathy for the union, saying the government should consider back-to-work legislation or replacement workers.
However, Paul Davidson, president of the Association of Universities and Colleges of Canada, wasn't taking sides even though he said the job action is taking its toll on foreign students, their families and Canadian post-secondary institutions that rely on business from abroad.
"Both sides have their strategies, have their tactics," he said in an interview.
Foreign students contribute about $8 billion annually to the Canadian economy.
Davidson couldn't estimate the overall cost of the strike as some students, their visas requests to Canada stalled, chose to study in other countries such as the United States or Britain.
"The concern is a reputational impact if the word gets out that it's awkward, difficult, uncertain about whether you'll get a visa from Canada. That will do a lot of damage to Canada's reputation."
Davidson said that although Treasury Board President Tony Clement is directly responsible for negotiations with the union, other federal cabinet ministers have a stake in the talks and should be seized with finding a solution.
He identified Foreign Affairs Minister John Baird, Trade Minister Ed Fast and Immigration Minister Chris Alexander.
"We are disappointed that the union has refused our reasonable offer," said Clement's spokesman Matthew Conway in an email.
He said the union "breached good faith bargaining principles by releasing confidential negotiating documents."
Last week the union publicly released confidential correspondence from Clement's office to demonstrate the preconditions that it was being asked to agree to.
Monday's job action took place in Beijing, Mexico City and New Delhi, London, Paris, Abu Dhabi and Shanghai, among others.
The union wants wage parity with counterparts in other federal departments who the union says earn as much as $14,000 more for doing similar work.