June 26 (Reuters) - Canada's WestJet Airlines said on

Wednesday a union representing its maintenance engineers had

rejected its new pay offer and served a second strike notice,

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 ( ACDVF ), has been facing demands from

employees for improved working conditions and higher salaries.

A work stoppage by the Aircraft Mechanics Fraternal

Association (AMFA) could occur as soon as June 28, the airline

said in a statement.

The offer that was rejected had presented aircraft

maintenance engineers with a 22% pay increase over four years.

"WestJet counter-proposed an economic enhancement of less

than 1% at the cost of an extended contract term -- the

effective affirmation of the company's previous public

statements that it would refuse to engage in meaningful

negotiations with respect to economic issues," AMFA said in a

statement.

AMFA also said it had agreed to extend negotiations in

Toronto through the end of this week and resume in Calgary next

week if necessary.

The airline last week canceled 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.