Airlines for America (A4A), which represents American Airlines, Delta Air Lines, United Airlines and others also asked the FAA to revise a proposed 5G safety directive "to reflect technical realities and the continued safe operation of many aircraft." The group warned a "material number of aircraft" in U.S. fleets will not be modified by July and without changes it could "severely limit operations" and lead to flight delays and cancellations.

The International Air Transport Association (IATA), which represents more than 100 carriers that fly to the United States, Thursday warned costs would be far higher than the $26 million estimated by the agency -- and said it could be at least $637 million. The group warned last week many airlines are at risk of not meeting the deadlines.

The FAA proposed in January requiring passenger and cargo aircraft in the United States have 5G C-Band-tolerant radio altimeters or approved filters by February 2024. The agency reiterated on Friday it will evaluate all comments.

Concerns that 5G service could interfere with airplane altimeters, which give data on a plane's height above the ground and are crucial for bad-weather landing, led to disruptions at some U.S. airports last year involving international carriers.

Verizon Communications Inc and AT&T Inc in June voluntarily agreed to delay some C-Band 5G use until July as air carriers work to retrofit airplanes to ensure they will not face interference.

"Global supply chain issues and lack of certified solutions will make total compliance with the proposed (directive) deadlines difficult, if not impossible to meet without significant adverse impacts on the public," A4A said.

Wireless carries spent more than $80 billion on the C-Band 5G spectrum, including $52.9 billion by Verizon in auction and clearing costs. The FCC, AT&T and Verizon did not immediately comment.