"Since the beginning of 2020 we’ve refunded over 11 million tickets totaling $6 billion, of which 20% has taken place in 2022," Bastian wrote Transportation Secretary Pete Buttigieg in a letter made public on Wednesday.

He also disclosed Delta automatically deposited an extra 10,000 miles in the frequent flyer accounts of travelers who experienced travel interruptions between May 1 and July 7.

Some in Congress have pressed airlines to do more to boost refunds. Buttigieg said recently the department has completed 10 air carrier investigations into refund practices and is pursuing enforcement actions and has proposed new rules on ticket refunds.

Bastian said "Delta’s 2022 capacity restoration compared to 2019 is the lowest among our competitors." He added Delta "currently have 91% of 2019 active pilots covering 86% of 2019 departures."

The Federal Aviation Administration disclosed earlier this month that in May, Delta pilots missed 19,985 days due to sickness, up 45% from 13,786 days in May 2019. In June, pilot sick days rose 50% from June 2019. Delta also reported in May its flight attendants missed 43,908 days due to sickness, up 23%, the FAA said.

The FAA on Aug. 12 said Delta could temporarily cut some flights at New York's LaGuardia and John F. Kennedy airports and Ronald Reagan Washington National Airport.

The FAA said as a condition of approval that Delta "should offer customers a refund or rebook them on Delta or another carrier as needed for canceled flights at the three airports."

In July, Democratic Senators Edward Markey, Elizabeth Warren, Sheldon Whitehouse and Richard Blumenthal along with nearly 20 House of Representatives lawmakers introduced legislation to provide consumers an "enforceable right to a full cash refund for flight and ticket cancellations."