The U.S. Federal Aviation Administration (FAA) informed that interference from 5G spectrum posed an air safety risk and could result in flight diversions.

The aviation industry and the FAA have raised concerns about potential interference of 5G services with sensitive aircraft electronics like radio altimeters.

Leading telecom operators such as AT&T and Verizon Communications in November agreed to delay the commercial launch of C-band wireless service until Jan. 5 after the FAA raised concerns.

The FAA issued a pair of airworthiness directives ordering the revision of airplane and helicopter flight manuals to prohibit some operations requiring radio altimeter data when in the presence of 5G C-Band wireless broadband signals.

One FAA directive on Tuesday said the unsafe condition posed by the planned use required immediate action before the Jan. 5 deployment because radio altimeter anomalies that are undetected by the aircraft automation or pilot, particularly close to the ground … could lead to loss of continued safe flight and landing.

The FAA reiterated in a statement on Tuesday that it believes the expansion of 5G and aviation will safely co-exist. The agency added that the two directives provide a framework … to gather more information to avoid potential effects on aviation safety equipment.

The FAA remains in discussions with the Federal Communications Commission (FCC), White House and industry officials about the precise contours of any limitations, which are expected to be outlined in the coming weeks in a series of notices.

The FCC said on Tuesday that “it continues to make progress working with the FAA and private entities to advance the safe and swift deployment of 5G networks … We look forward to updated guidance from the FAA in the coming weeks that reflects these developments.”

It is not yet clear what airports or specific airplanes may be impacted. The FAA said notices would be issued, as necessary, to state the specific areas where the data from a radio altimeter may be unreliable due to the presence of 5G C-Band wireless broadband signals.

AT&T and Verizon on Nov. 24 said they would adopt precautionary measures for at least six months to limit interference. But aviation industry groups said on Monday they were insufficient to address air safety concerns.

Verizon said on Tuesday there is no evidence that 5G operations using C-band spectrum pose any risk to aviation safety, as the real-world experience in dozens of countries already using this spectrum for 5G confirms. Verizon is confident the FAA ultimately will conclude C-Band 5G use poses no risk to air safety.

Verizon said it was on track to launch 5G using C-band next month and to reach 100 million Americans with this network in the first quarter of 2022.

The wireless companies said in November they would take additional steps to minimize energy coming from 5G base stations. The FAA said under 2020 FCC rules base stations in rural areas of the United States are permitted to emit at higher levels in comparison to other countries.