WASHINGTON, Dec 15 (Reuters) - United Airlines (UAL.O) Chief Executive Scott Kirby on Wednesday defended a decision to mandate COVID-19 vaccines for workers amid criticism from some Republican senators. "We did this for safety. We believe it saved lives," Kirby said at a Senate Commerce hearing on aviation issues. "We don"t compromise on safety." Republican Senator Ted Cruz criticized United"s vaccine mandate, calling it "disturbing" and was "disregarding the rights of your employees." Cruz noted American Airlines (AAL.O) and Southwest Airlines (LUV.N) both have said they do not plan to fire any employee over the vaccine requirements. Kirby said about 200 employees did not comply with United"s mandate and were fired of its 67,000 employees. He added about 6 pilots were fired and 80 on unpaid leave out of about 13,000. On Monday, a panel of the 5th U.S. Circuit Court of Appeals rejected an emergency request for an injunction blocking the mandate while the employees appeal a November ruling by a federal judge in favor of United. United Airlines was the first major air carrier to issue a vaccine requirement and others followed. United granted around 2,000 religious and medical exemptions to employees. Kirby said about 80% of those seeking religious exemptions were granted. The wide ranging aviation hearing covered a number of issues. Airlines said they were ramping up hiring and struggling to find enough pilots. Kirby said United effectively had nearly 100 regional airplanes grounded "because there"s not enough pilots to fly them.... The country is going to need thousands of pilots." Southwest Chief Executive Gary Kelly said at the hearing: "I think the case is very strong that masks don"t add much if anything in the air cabin environment -- it"s very safe, very high quality compared to any other indoor setting." Republican Senator Roger Wicker asked when passengers will be able to stop wearing masks. This month, the Biden administration extended transportation mask requirements through March 18. Sara Nelson, president of the Association of Flight Attendants-CWA, said lifting mask requirements was up to medical experts but added "we absolutely look forward to the day that we no longer have the mask requirement."