(Reuters) -Canada’s WestJet Airlines said on Wednesday a union representing its maintenance engineers had rejected its new offer and served a second strike notice, just days after the union called off a strike and agreed to return to the bargaining table.

WestJet, which is backed by Onex Corp and competes with Air Canada, has been facing demands from employees for improved working conditions and higher salaries.

A work stoppage by the Aircraft Mechanics Fraternal Association could occur as soon as June 28, the airline said in a statement.

Its latest offer had presented a revised pay that would see aircraft maintenance engineers receive a 22% increase over four years, the carrier said.

The union didn’t immediately respond to a Reuters’ request for comment.

The airline last week cancelled 40 flights that affected 6,500 customers after it was rendered a 72-hour strike notice by the union. On June 20, the union withdrew the strike notice and agreed to return to discussions.

(Reporting by Kanjyik Ghosh; Editing by Sonia Cheema and Mrigank Dhaniwala)