US telecom giants AT&T and Verizon on Sunday sharply rebuffed a request from US authorities to again postpone their rollout of 5G networks to allow more study of possible interference with flight safety equipment.

The US introduction of the high-speed mobile broadband technology had been set for December 5, but was delayed to January 5 after aerospace giants Airbus and Boeing raised concerns that the new system might interfere with the devices planes use to measure altitude.

US Transportation Secretary Pete Buttigieg and the head of the Federal Aviation Administration (FAA), Steve Dickson, asked for the latest delay in a letter sent Friday to AT&T and Verizon.

After studying the request, the two telecom operators bluntly turned it down.

"Agreeing to your proposal would not only be an unprecedented and unwarranted circumvention of the due process and checks and balances carefully crafted in the structure of our democracy, but an irresponsible abdication of the operating control required to deploy world-class and globally competitive communications networks," they said in a joint letter.

Those networks, it added, "are every bit as essential to our country’s economic vitality, public safety and national interests as the airline industry."

The letter, seen by AFP, was signed by CEOs John Stankey of AT&T and Hans Vestberg of Verizon.

They blamed the government's last-minute request on what they suggested was a delay by the aviation sector in fully studying the impact of 5G on aircraft.

Last February, Verizon and AT&T were authorized to start using 3.7-3.8 GHz frequency bands as of December 5, after obtaining licenses worth tens of billions of dollars.

But when Airbus and Boeing raised their concerns about possible interference with airplanes' radio altimeters -- which can operate at the same frequencies -- the launch date was pushed back to January.

The FAA requested further information about the instruments, and it issued directives limiting the use of altimeters in certain situations, which sparked airline fears over the potential costs.

The AT&T and Verizon chiefs did hold out a possible olive branch, however, saying they remain "committed to continue our cooperation with your Department and all interested parties... on the condition that the FAA and the aviation industry are committed to doing the same without escalating their grievances."

The CEOs' letter also pointed to France's experience in reconciling 5G and concerns of the aviation sector.

"US aircraft currently fly in and out of France every day with thousands of US passengers and with the full approval of the FAA," they wrote.

"As a result, France provides a real-world example of an operating environment where 5G and aviation safety already co-exist."

France's civilian aviation authority, however, recommended in February that cell phones using 5G technology be kept off to avoid interference with planes' radio altimeters that could, it said, cause "critical" errors during landing.

It also imposed limits on the power of 5G antennas located near certain airports.