US authorities have asked telecom operators AT&T and Verizon to delay for up to two weeks their already postponed rollout of 5G networks amid uncertainty about interference with vital flight safety equipment.

The US rollout 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 about potential interference with the devices planes use to measure altitude.

US Transportation Secretary Pete Buttigieg and the head of the Federal Aviation Administration, Steve Dickson, asked for the latest delay in a letter sent Friday to AT&T and Verizon, two of the country's biggest telecom operators. 

The US letter asked the companies to "continue to pause introducing commercial C-Band service" -- the frequency range used for 5G -- "for an additional short period of no more than two weeks beyond the currently scheduled deployment date of January 5." 

The companies did not immediately respond to a request for comment.

The US officials' letter assures the companies that 5G service will be able to begin "as planned in January with certain exceptions around priority airports."

The officials say their priority has been "to protect flight safety, while ensuring that 5G deployment and aviation operations can co-exist."

Last February, Verizon and AT&T were authorized to start using 3.7-3.8 GHz frequency bands on 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 in 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.

When Verizon and AT&T wrote to federal authorities in November to confirm their intention to start deploying 5G in January, they said they would take extra precautions beyond those required by US law until July 2022 while the FAA completes its investigation.

The conflict between 5G networks and aircraft equipment led French authorities to recommend switching off mobile phones with 5G on planes in February.

France's civil aviation authority said interference from a signal on a nearby frequency to the radio altimeter could cause "critical" errors during landing.