Southwest Airlines Co. planes stand on the tarmac at San Francisco International Airport (SFO) in San Francisco, California, U.S., on Friday, Jan. 19, 2018. Southwest Airlines Co. is scheduled to release earnings on January 25. Photographer: David Paul Morris/Bloomberg via Getty Images

Southwest Airlines told its mechanics on Friday that it is experiencing an "operational emergency" due to an unusually high number of grounded jets -- and demanded they show up for work or risk termination, according to a company memo issued on Friday and seen by CNBC.

On Friday, 100 Southwest flights were cancelled, more than any other U.S. airline, according to flight-tracking site FlightAware, and more than 1,000 were delayed.The airline usually plans for having as many as 20 aircraft removed from service for unexpected maintenance issues every day.

However, each day this week, the percentage of out-of-service aircraft among its available fleet of about 750 Boeing 737s has been double the daily average, "with no common theme among the reported items," the airline said in statement.

"To take care of our customers, we are requiring all hands on deck to address maintenance items so that we may promptly return aircraft to service," Southwest's statement said. "At the same time, our operational planners have been working in the background to minimize the impact to our Customers."

The airline -- which has been in contract talks with its mechanics since 2012 -- told mechanics that if they are "alleging illness" they must provide a doctor's note when they return to work or risk losing their jobs, according to the memo.

"The uptick in maintenance items we experienced over the last few days have resulted in a slight increase" in cancellations, a Southwest spokeswoman told CNBC, but declined to provide a breakdown between disruptions caused by either maintenance or weather. On Saturday, 39 Southwest flights were canceled, more than other airlines, according to FlightAware.

Southwest told the mechanics it would assign them overtime, and would only honor vacation or shift trade requests that had already been approved, according to the memo, which was first reported by the Chicago Business Journal.

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