Striking Air Canada flight attendants vowed on Monday to defy another back-to-work order from the country's labour tribunal, but resumed talks seeking to end a walkout that has cancelled travel for half a million people worldwide.

Following federal labour minister Patty Hajdu's intervention to halt the strike over the weekend, the Canada Industrial Relations Board (CIRB) ordered the flight attendants back to work on Sunday. But the flight attendants' union said it would defy the order, forcing Air Canada to walk back plans to partially restore service.

CIRB regulators upped the pressure on Monday and ordered the union "to resume the performance of their duties immediately and to refrain from engaging in unlawful strike activities," Air Canada said in a statement.

The tribunal gave the Canadian Union of Public Employees (CUPE) until 12:00pm to communicate to members that they "are required to resume the performance of their duties," the carrier added.

Speaking after that deadline, CUPE President Mark Hancock told reporters the solution "has to be found at a bargaining table," and that the union will not respect the tribunal's ruling.

"None of us want to be in defiance of the law," he said, but stressed the union would not waver in advocating for people asked to work hours on the ground during flight delays without "getting paid a dime."

If Air Canada "thinks that planes will be flying this afternoon, they're sorely mistaken," Hancock said.

The union said later on Monday that it had resumed talks with the airline as part of "continued attempts to reach a fair deal." (AFP)