A pair of U.S. senators have reached out to as many as 10 airlines requesting that they "commit" to providing refunds for flights canceled amid the COVID-19 pandemic or "at a minimum" eliminate any expiration dates for future flight credits.

According to a letter obtained by CNN , Sens. Ed Markey and Richard Blumenthal, two Democrats from Massachusetts and Connecticut, respectively, cited an estimate that U.S. airlines, which have received billions of dollars in grants and loans over the past year, are currently holding on to $10 billion in outstanding and recently expired flight credits. However, Consumer Reports estimates that the number is between $12 billion and $15 billion.

"Commit to providing a cash refund for all tickets that are canceled during the coronavirus pandemic," the senators urged. "Americans need cash in their pockets to pay for food, housing, and prescriptions during this emergency. Without removing expiration dates, your company may be encouraging travelers to fly before they feel safe boarding a plane, lest they lose tickets that they have already purchased with hard-earned dollars."

Refunds have become by far the biggest complaint from U.S. air travelers since the pandemic was declared 14 months ago. According to Department of Transportation data, passengers filed more than 107,000 official complaints since March 2020, 89 percent of which have been about refunds. Some carriers have extended credit expiration dates into next year but Consumer Reports Advocacy aviation adviser Bill McGee told CNN that policies "are extremely complicated. Quite frankly, why should there be any expiration on vouchers in the age of COVID?"

The airlines contacted by Markey and Blumenthal include Alaska Airlines, Allegiant Air, American Airlines, Delta Air Lines, Frontier Airlines, Hawaiian Airlines, JetBlue Airways, Southwest Airlines, Spirit Airlines and United Airlines.