New documents obtained by CBS News show the situation at Southwest Airlines is worse than previously known. At different times throughout the last week, more than 60 Southwest 737s were taken out of service by mechanics for unscheduled maintenance. The airline says the "unscheduled aircraft downtime," or UADs, have spiked at several maintenance hubs in particular.

Southwest said that since Feb. 12, the average UAD in Orlando has surged to 60 hours a day compared to an average of 10.2 hours per day over the last two years. In Houston, the UAD had averaged 18.6 hours a day but jumped to 111 hours Feb. 12, then rose again to 127 the following day.

"Yet there have been no mechanical issues on Southwest justifying this level of OTS aircraft," Southwest executive vice president and chief legal and regulatory officer, Mark Shaw, wrote in a letter to the mechanics union Friday.

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He accused the union of organizing an illegal worker action to protest stalled contract negotiations.

"We have identified a group of approximately 100 Mechanics who are responsible for virtually the entire increase in UAD hours ... This concentration of activity in a discernible pattern makes clear that the source of the increase… is an unlawful concerted activity," Shaw wrote.

Bret Oestreich, the national director of the American Mechanics Fraternal Association, said the union — which represents about 2,600 Southwest mechanics — "firmly rejects these allegations."

"AMFA has not called for, does not support, and will oppose any job action, in any form," he wrote in a letter to the mechanics on Friday in response to Shaw's letter. "Members are unequivocally instructed to refrain from any collective actions to withhold their services from the Company, or to diminish their services, or to disrupt operations for illegal reasons. Doing your job as a licensed Technician is not illegal."

Nick Granath, a lawyer representing the mechanics union, said Southwest and its senior leadership "should be ashamed."

"AMFA members at Southwest Airlines are doing their jobs in accordance with the requirements of their FAA-issued A&P licenses," he said. "Southwest Airlines should be thanking these men and women for their dedication to safety; instead, it hurls unfounded accusations."

The escalating war of words between the airline and union come as Southwest CEO Gary Kelly tried to strike a conciliatory tone in an email to employees late Friday — his first official comments since the surge in out-of-service aircraft, which has resulted in at least hundreds of canceled flights and thousands of delays.

Southwest, which operates a fleet of about 750 Boeing 737s, typically has as many as 20 planes out of service at any given time. The airline previously indicated earlier this month it had seen that increase into the 40s on a daily basis, before the number jumped to 60 last week.

"We suddenly find ourselves in a period of tension and turmoil surrounding out-of-service aircraft for maintenance and AMFA contract negotiations," Kelly wrote in his email, apologizing to employees and customers for "hardship" the situation has created over the last two weeks. "Our Mechanics are extraordinary. I am proud of them, and they have been especially heroic in getting aircraft returned to service over the last two weeks. They deserve all of our thanks. They also deserve a new labor contract."

Earlier this month, a CBS News investigation discovered allegations by mechanics at Southwest of undue pressure to keep airplanes in service. We've since heard from more than two dozen Southwest mechanics who say coworkers were emboldened to write up maintenance issues after our report aired in an effort to fully comply with FAA regulations.

Southwest Airlines and the AMFA have been locked in tense contract negotiations for six years. Last fall, the mechanics voted down a new contract, and the airline says the current operational issue began almost immediately after the latest bargaining sessions. Mediated labor negotiations are set to begin March 12.

The recent "unprecedented increase" in out-of-service aircraft prompted Southwest to declare an "Operational State of Emergency" at five of its 20 maintenance facilities — Dallas, Houston, Las Vegas, Orlando and Phoenix. The emergency declaration allows the airline to assign additional work hours, change shifts and assignments, limit vacations and shift swaps as well as expand its use of third-party vendor mechanics.

Out-of-service aircraft have been hindering the airline's operation for over a week now, leading to hundreds of cancellations and thousands of delays. On Saturday, Southwest continued to experience more delays and cancellations than any other U.S. carrier, according to FlightAware.

But while hundreds of mechanics have signed up to work overtime at the five maintenance bases where Southwest Airlines has declared an operational state of emergency, the airline told CBS News it has "not had a need to lean on overtime." According to Aircraft Mechanics Fraternal Association, only two mechanics — in Phoenix — were called to work overtime in the midst of the "so-called 'operational emergency.'"

Southwest spokeswoman Brandy King told CBS News via email when that because of the operational emergency "only a limited number of mechanics are utilizing vacation days" and the airline has "reallocated additional staffing to support scheduled maintenance and out-of-service aircraft" resulting in "more hands on deck" to do the needed work, reducing the need for overtime.

When the initial state of emergency was announced on February 15, the union expressed concern about the possibility of mandatory overtime leaving workers fatigued. Southwest said it has not needed to "activate" mandatory overtime, allowing the airline "to ensure we keep our Employees safe and not introduce unnecessary fatigue."

Southwest is steering some of the work away from its own mechanics to third-party contractors instead.

Chief Operating Officer Mike Van de Ven said in a statement Tuesday the airline was using a team of third-party vendors to handle as "much scheduled maintenance program work … as possible which allows our Southwest mechanics to work the increased workload of maintenance tasks they have identified."

Van de Ven appeared to blame the union for the out-of-service aircraft, writing in a statement, "AMFA has a history of work disruptions, and Southwest has two pending lawsuits against the union. We will be investigating this current disruption and exploring all possible remedies."

AMFA's National Director Bret Oestreich responded, calling those claims "scapegoating" and "simply an attempt to divert attention away from the airline's safety issues."

The union told CBS News, "All of the out of service aircraft were written up for legitimate problems."