The US Department of Transportation (DOT) has announced it will conduct an extraordinary audit into the Federal Aviation Administration's (FAA) safety oversight of Allegiant Air (G4, Las Vegas Harry Reid) and American Airlines (AA, Dallas/Fort Worth), Reuters has reported.

The audit of the safety oversight exercised in relation to Allegiant Air follows a report on CBS' "60 Minutes" aired in mid-April 2018 which alleged widespread safety violations at the airline. Following the programme, US Senator Bill Nelson called for an investigation.

It is not immediately clear why the DOT decided to extend the audit to cover American Airlines.

"American Airlines was shocked to learn of the Office of Inspector General’s review and we stand by our strong safety record. Our team is working to understand why we are part of its review," spokesman Ross Feinstein has said in a statement.

Both carriers have said they welcome the audit as they are fully committed to safety.

In a memo, the DOT has said it has decided to initiate the audit after learning that the FAA had earlier adopted a policy moving away from enforcement actions and focusing on working with carriers to address the root causes of identified violations. Reportedly, the DOT has also found "significant discrepancies" in the relationship between various airlines and the FAA offices.