(Reuters) -Canada's WestJet Airlines has begun cancelling flights again after a union representing its maintenance engineers rejected a new pay offer and served a second strike notice, the carrier said on Wednesday.

WestJet said this action would lead to an estimated 25 flight cancellations from June 27-28 and impact over 3,300 travellers. The airline had earlier in the day said that the work stoppage could occur as soon as June 28.

It is not clear how long the strike could go on for.

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

WestJet said it presented the Aircraft Mechanics Fraternal Association (AMFA) with a "Canadian industry-leading agreement," which was better than the previous agreement that was rejected by the union earlier in the day.

The offer that was rejected had presented aircraft maintenance engineers with a 22% pay increase over four years.

AMFA did not immediately respond to a request for comment.

"It is especially devastating that the strike notice we have received from AMFA forces us to begin cancelling flights and parking aircraft, for the second time in just over a week," said Diederik Pen, president of WestJet Airlines and group chief operating officer.

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