June 28 (Reuters) - Aircraft maintenance engineers and

other technical operations employees at Canada's WestJet

Airlines went on strike at the start of a long weekend on

Friday, after rounds of negotiation failed to reach agreement on

pay and working conditions.

WestJet expressed outrage at the strike during the Canada

Day weekend, one of the country's most important holidays,

saying it would hold the union "100% accountable for the

unnecessary stress and costs incurred as a result."

Labour Minister Seamus O'Regan, seeking to avert the strike,

had intervened on Thursday after WestJet began canceling

flights, ordering Canada's industrial relations board to impose

final binding arbitration in the dispute.

The Aircraft Mechanics Fraternal Association (AMFA) defended

the strike in a statement on Friday, saying O'Regan's order was

silent on "employees' fundamental right to strike," as

guaranteed by the Canadian Charter of Rights and Freedoms, so

"AMFA members' constitutional right to strike must prevail."

The union said it had served a strike notice on June 18

after 97% of its members voted to reject a tentative pay deal

reached in May but that negotiators returned to the bargaining

table two days later.

That round also failed, and the union served a second strike

notice, with employees walking off the job at 7:30 p.m. ET (2330

GMT), AMFA said.

Calgary-based WestJet, which is backed by Onex Corp ( ONEXF )

and competes with Air Canada ( ACDVF ), said severe

travel disruption is to be expected if the strike is not called

off immediately.

The airline said it was actively pursuing measures to

minimize flight disruption, including asking for immediate

action by the Canadian government.

AMFA said the airline had told the union it would not engage

in further collective bargaining in Toronto, citing the

government-imposed arbitration. The airline also rejected a

request for continuation of collective bargaining in Calgary

next week, the union said.

The union said its bargaining committee was standing by to

continue discussions with the airline, adding they were

available to resume collective bargaining over the weekend and

into next week.