Air Canada says it is facing "a number of operational challenges" during one of the busiest travel days of the year after warning earlier this week that pilots could disrupt service by calling in sick or reporting fatigue.

Websites for both the airline and the airports list a number of delays and cancellations for flights between Montreal and other cities including Toronto, Halifax, Saint John, N.B., and some American destinations.

They come just days after the airline issued a statement saying it believes the Air Canada Pilots Association was planning "to engage in illegal job action in the form of … increased sick calls" amid contract disputes with pilots, ground crews and baggage handlers.

Air Canada spokesman Peter Fitzpatrick wrote in an email to CBC News on Saturday that the airline could only say it was facing "a number of operational challenges," including weather, during what is considered to be a peak travel weekend at the tail end of the March break.

Fitzpatrick did not confirm reports that an unknown number of pilots had called in sick on Saturday, saying only the "vast majority of Air Canada employees are working hard to get our customers to destination safely."

The pilots' association was not immediately available for comment, but some pilots took to Twitter to say the weather was the cause of any service disruptions, not sick calls by co-workers, the CBC's Karina Roman reported.

Tories push through back-to-work legislation

Earlier this week, the Conservative government's back-to-work bill to send a pair of Air Canada contract disputes to binding arbitration passed the Senate.

Labour Minister Lisa Raitt proposed the back-to-work legislation, which would cover about 8,600 mechanics, baggage handlers and other ground crew and about 3,000 pilots. Raitt said the move was necessary to avoid a work stoppage that would harm the economy.

When contacted by CBC News about Saturday's delays and cancellations, the minister's office said it was aware of the situation.

"These issues fall with Air Canada internally, and should Air Canada feel that these actions constitute an illegal strike, they can bring this matter to the Canadian Industrial Resolution Board," Raitt said in a statement.

Chair of the pilots association's flight safety division, Barry Wiszniowski, sent out a memo on March 8 to union members, reminding them of their professional responsibility to consider how "accumulated stress" might negatively affect their alertness and flight performance.

"As you are well aware, there have been recent events that may have increased your personal stress levels both at work and at home with your family," Wiszniowski wrote. "It is important that you understand that as a professional pilot, you have a duty and obligation to ensure that when you operate an aircraft you must be fit for the task at hand."

In another development, Jean-Marc Bélanger, chair of the master executive council of the pilots association, wrote in an email to fellow Air Canada pilots on Saturday that he was "booked off" flying duty "because I have self-assessed as unfit to fly."

Bélanger cited sleep deprivation "and the unknown commitment of the corporation to support me in line with the many aspects of our joint obligations" as reasons for deciding he could not fly.