New York CNN  —  Labor negotiations between WestJet, Canada’s second-largest airline, and the Aircraft Mechanics Fraternal Association remain at a standstill after the union called a surprise strike last week, forcing more than 400 flight cancellations and delays over the weekend. The union, which represents nearly 700 aircraft engineers and technicians, went on strike Friday after negotiations over the group’s first collective bargaining agreement with the airline broke down over what it said was WestJet’s “refusal to negotiate.” The strike follows back-and-forth talks between the union and the airline stretching back to September 2023, when representatives met for an initial bargaining session. At the meeting, according to a memo from the association, union representatives raised concerns including “furloughs out of seniority order, irregularities in pay scale, and work privileges that have been eliminated without notice or rationale.” In addition to wage increases, the union is seeking protections for certain positions and improved benefits such as health care. On Sunday, the union told CNN “the parties are separated by a first-year economic difference of approximately 7%, less than $8 million.” WestJet has repeatedly condemned the strike, which comes during a long holiday weekend in Canada. The company says the strike will affect about 250,000 travelers’ plans. WestJet said in a statement Friday it has sought “immediate intervention by the Minister of Labour and the Canada Industrial Relations Board.” The airline says it has had to cancel more than 410 flights between Thursday and Saturday. Data from flight tracker FlightAware shows WestJet on Sunday had canceled at least 343 flights, constituting 77% of its fleet. The airline’s regional subsidiary, WestJet Encore, has canceled at least 80 flights. Airports across Canada are also facing major service suspensions. At Calgary International Airport, 42% of outbound flights and 40% of inbound flights have been canceled. Toronto Pearson, Vancouver and Edmonton international airports are also affected. But in an update to members on Sunday shared with CNN, the union announced the industrial relations board ruled the strike was lawful, as the board continues to mediate negotiations. “Although the compulsory arbitration imposed by the Minister of Labour is scheduled to move forward on an expedited basis, the CIRB has held that, until that process is completed, we are entitled to continue our legal strike,” the union said. The industrial relations board has not responded to CNN’s request for comment. The union told CNN it had made its last offer to the airline on Thursday, and since then WestJet has not responded or given any indication of future bargaining dates. The union also detailed a drawn out and unproductive mediation process over the weekend. “At 1:00 a.m. Eastern on Saturday, June 29, after waiting hours in a virtual breakout room, the mediators returned to inform us that WestJet executives had decided to ‘call it a night,’” the union said in its update. Both parties and federal mediators are set to continue talks into Sunday.