(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 ( ONEXF ) 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)