U.S. regulators are warning aviators that a new band of 5G mobile phone service might interfere with key safety devices on aircraft.

The Federal Aviation Administration on Tuesday issued a Special Airworthiness Information Bulletin warning that “action might be required to address potential interference with sensitive aircraft electronics.”

The 5G spectrum abuts radio signals used by so-called radar altimeters, which measure how close an aircraft is to the ground.

While FAA took pains to say it is working with the Federal Communications Commission and other agencies to allow the new technology to safely coexist with aviation, the safety alert creates an unusual situation in which one agency raises concerns while another has granted its approval. It also illustrates growing frustration within the aviation industry.

Canada recently imposed restrictions on locating new 5G cell towers near the runways of large airports. Australia, France and other nations have taken steps to limit the chances of aircraft interference.

Radar altimeters are used on planes and helicopters for multiple critical safety functions, including landing when visibility is low, anti-collision warnings and systems that warn pilots when they inadvertently get too low. Some commercial helicopter flights can’t operate without a working radar altimeter.

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The FAA bulletin said pilots should remind passengers to place any 5G device into airplane mode or switch them off during flight, and to notify the agency of any signs of interference.

The new 5G spectrum, called C-Band, can become operational on Dec. 5. The FCC awarded wireless network providers access to the radio bands in a February auction.

“The FCC is committed to continuing to work with its federal partners to simultaneously preserve air safety and advance the deployment of new technologies that promote American consumer and business needs,” the agency said in a statement.

CTIA, a trade group representing the wireless industry, said that active 5G networks using the same spectrum band work safely in 40 countries. The group said the issue has been studied by agencies around the world, including the FCC, who’ve considered submissions from the aviation industry.

“5G networks using C-band spectrum operate safely and without causing harmful interference to aviation equipment,” the group said in a statement. “C-band spectrum is critical to delivering 5G service in communities large and small across the country, ensuring all Americans benefit from these next generation networks. Any delay in activating this spectrum risks America’s competitiveness and jeopardizes our ability to ensure global 5G leadership.”

RTCA Inc., a Washington-based nonprofit that studies technical aviation issues, in a report last year concluded that the potential for interference created a safety hazard. It found “significant impacts throughout the approach with the potential for catastrophic effects.”

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The FAA bulletin said there have been no confirmed reports of interference.

The issue is that the FAA’s technical standards for radar altimeters were crafted years before the potential for mobile-phone companies to use nearby frequencies arose. As a result, tens of thousands of the devices are in use without any protection against adjacent radio waves, the FAA said in the bulletin.

In comments to the FCC, aviation industry representatives have said that it would take years to develop new standards for radar altimeters and then replace or upgrade them.